Stabilize MCP timeout lease test under full-suite load

TESTING_GUIDE.md

@@ -509,6 +509,7 @@ Use `docs/test-doc-maintenance-plan.md` before moving oversized suites or adding
   Keep `PathCasing.IgnoreCaseProbeForTesting` execution-context-local, and never write an injected cache-miss result into the shared path-casing cache. Existing production-seeded cache entries still take precedence; injected probe outcomes must flow to child tasks owned by that test without leaking into concurrently running LSP or other test classes through either the delegate or cached values.
 - `McpServer*Tests.cs`
   MCP JSON-RPC behavior and tool outputs. Large server coverage is split into focused partial suites for tool calls, tool listing, protocol/session handling, and error handling while the root `McpServerTests` part keeps shared seeded fixture state. Request-timeout tests use signal-gated delay hooks instead of fixed sleeps: start the request, confirm the hook has begun, then await the timeout response with a bounded wait so they pay only the configured timeout while still proving in-flight actions drain after the timeout response.
+  The single-request timeout-lease regression uses an ID-specific dispatch signal, a one-second execution timeout for scheduler headroom, typed response-node assertions, and `TestDeterminism.AssertTaskRemainsBlockedAsync` for the queued request. Keep those checks together so full-suite load produces an actionable assertion instead of a null dereference (#4807).
   The root seeded database/server fixture is initialized through a thread-safe `Lazy<T>` only when a test accesses its default fixture path. Static helpers and tests that build their own server or transport must not pay schema creation and seed cost; concurrent fixture access must still publish exactly one database/server pair.
   Protocol negotiation coverage keeps `2025-06-18`, `2025-03-26`, and `2024-11-05` in one shared version-echo fixture. The Codex compatibility regression separately uses the lifecycle-enforcing transport to send a `2025-06-18` initialize, `notifications/initialized`, and `tools/list`, because a direct handler assertion would not catch initialization-gate failures.
   Request-id telemetry coverage uses credential-shaped and high-cardinality ids to prove raw values never reach stderr prefixes/events, Activity tags, MCP metrics, audit records, or timeout logs/status. Assert the fixed opaque token plus consistent type and decoded string-value UTF-16 code-unit length (`null` = `0`), different injected process salts, JSON type domain separation for equal textual values, and collapse to one overflow token after the distinct-id budget (including concurrent creation). Keep a CLI metrics negative case that omits all request-id fields.
@@ -1356,6 +1357,7 @@ dotnet test --filter "FullyQualifiedName~GitHelperTests"
   `PathCasing.IgnoreCaseProbeForTesting` は execution context ごとに隔離し、注入した cache-miss 結果を共有 path-casing cache へ書き込まないでください。既存の production seed 済み cache entry は引き続き優先します。注入した probe 結果はその test が所有する child task には引き継ぎますが、delegate と cache のどちらからも並列実行中の LSP や他の test class へ漏らしてはいけません。
 - `McpServer*Tests.cs`
   MCP の JSON-RPC 挙動とツール出力のテスト。大きな server coverage は tool call、tool listing、protocol/session handling、error handling ごとの focused partial suite に分割し、共有の seed 済み fixture 状態は root 側の `McpServerTests` に残します。request-timeout test は固定 sleep ではなく signal-gated delay hook を使います。request を開始し、hook が始まったことを確認してから timeout response を bounded wait で待つことで、timeout response 後に in-flight action が drain されることは保ったまま、設定した timeout 分だけを待つようにします。
+  single-request の timeout-lease 回帰テストでは、ID 別の dispatch signal、scheduler の余裕を確保する 1 秒の execution timeout、型付き response-node assertion、queue 待ち request に対する `TestDeterminism.AssertTaskRemainsBlockedAsync` を使います。full-suite 負荷でも null 参照ではなく対応可能な assertion を返すよう、これらの検証をまとめて維持してください（#4807）。
   root の seed 済み database/server fixture は、test が既定 fixture path へアクセスした場合だけ thread-safe な `Lazy<T>` で初期化します。static helper や独自 server / transport を構築する test は未使用 schema の作成・seed cost を支払わず、並行 fixture access でも database/server pair を必ず1組だけ公開してください。
   protocol negotiation coverage は `2025-06-18`、`2025-03-26`、`2024-11-05` を共通の version-echo fixture にまとめます。Codex 互換性の回帰テストでは別途 lifecycle を強制する transport を使い、`2025-06-18` の initialize、`notifications/initialized`、`tools/list` までを送ります。direct handler の assertion だけでは initialization gate の失敗を検出できないためです。
   request-id telemetry coverage では credential 風および high-cardinality な id を使い、生値が stderr の prefix / event、Activity tag、MCP metrics、audit record、timeout log / status のどこにも出ないことを検証します。固定長 opaque token と、型および decode 後の string 値の UTF-16 code unit 数（`null` は `0`）が全 surface で一致することに加え、注入した process salt ごとの差、text が同じ JSON type 間の domain separation、concurrent creation を含む distinct-id budget 超過後の単一 overflow token への集約を確認してください。CLI metrics では request-id field をすべて省略する negative case を維持してください。

changelog.d/unreleased/4807.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 4807
+affected:
+  - tests/CodeIndex.Tests/McpServerTests.cs
+  - TESTING_GUIDE.md
+---
+
+## English
+
+- **MCP timeout-lease coverage no longer null-dereferences under full-suite load (#4807)** — the regression test now uses an ID-specific readiness signal, scheduler headroom, a bounded blocked-state observation, and typed JSON response assertions so concurrency regressions produce actionable test failures.
+
+## 日本語
+
+- **MCP timeout-lease のカバレッジが full-suite 負荷時に null 参照しないようになりました（#4807）** — 回帰テストで ID 別の readiness signal、scheduler の余裕、上限付きの blocked-state 観測、型付き JSON response assertion を使い、並行処理の回帰時に対応可能なテスト失敗を返すようにしました。

tests/CodeIndex.Tests/McpServerTests.cs

@@ -7031,7 +7031,7 @@ public async Task ProcessFrameAsync_TimedOutActionRetainsConcurrencyLeaseUntilIt
             toolFilter: null,
             maxConcurrency: 1)
         {
-            RequestTimeout = TimeSpan.FromMilliseconds(100),
+            RequestTimeout = TimeSpan.FromSeconds(1),
         };
         Assert.NotNull(await server.ProcessFrameAsync(
             """{"jsonrpc":"2.0","id":"issue-4536-timeout-init","method":"initialize","params":{}}"""));
@@ -7053,27 +7053,32 @@ public async Task ProcessFrameAsync_TimedOutActionRetainsConcurrencyLeaseUntilIt
                 return releaseFirst.Task;
             }
 
-            secondStarted.TrySetResult();
+            if (id?.GetValue<int>() == 453652)
+                secondStarted.TrySetResult();
             return Task.CompletedTask;
         };
 
         var firstResponseTask = server.ProcessFrameAsync(
             """{"jsonrpc":"2.0","id":453651,"method":"ping"}""");
         await firstStarted.Task.WaitAsync(TestDeterminism.DefaultTimeout);
         var firstResponseText = await firstResponseTask.WaitAsync(TestDeterminism.DefaultTimeout);
-        Assert.Equal("Request timed out", JsonNode.Parse(firstResponseText!)!["error"]!["message"]!.GetValue<string>());
+        var firstResponse = Assert.IsType<JsonObject>(JsonNode.Parse(Assert.IsType<string>(firstResponseText)));
+        var firstError = Assert.IsType<JsonObject>(firstResponse["error"]);
+        Assert.Equal("Request timed out", Assert.IsAssignableFrom<JsonValue>(firstError["message"]).GetValue<string>());
         Assert.Equal(0, server.AvailableConcurrencySlotsForTests);
 
         var secondResponseTask = server.ProcessFrameAsync(
             """{"jsonrpc":"2.0","id":453652,"method":"ping"}""");
         await secondRegistered.Task.WaitAsync(TestDeterminism.DefaultTimeout);
-        Assert.False(secondStarted.Task.IsCompleted);
+        await TestDeterminism.AssertTaskRemainsBlockedAsync(secondStarted.Task);
         Assert.Equal(0, server.AvailableConcurrencySlotsForTests);
 
         releaseFirst.TrySetResult();
         await secondStarted.Task.WaitAsync(TestDeterminism.DefaultTimeout);
         var secondResponseText = await secondResponseTask.WaitAsync(TestDeterminism.DefaultTimeout);
-        Assert.Equal("ok", JsonNode.Parse(secondResponseText!)!["result"]!["status"]!.GetValue<string>());
+        var secondResponse = Assert.IsType<JsonObject>(JsonNode.Parse(Assert.IsType<string>(secondResponseText)));
+        var secondResult = Assert.IsType<JsonObject>(secondResponse["result"]);
+        Assert.Equal("ok", Assert.IsAssignableFrom<JsonValue>(secondResult["status"]).GetValue<string>());
         Assert.Equal(1, server.AvailableConcurrencySlotsForTests);
     }