[codex] Fix LSP cancellation diagnostics and navigation semantics

[Widthdom]

Summary

• Thread LSP stdio frame reads through cancellation and bounded header buffers for LSP server should support cancellation and bounded header parsing #3427.
• Add structured LSP lookup failure diagnostics for safe empty definition/reference results in LSP lookup failures should return structured diagnostics instead of silent nulls #3428.
• Preserve additional LSP client semantics for LSP navigation and symbol responses should preserve client semantics #3537: declaration/typeDefinition/implementation aliases, includeDeclaration, all ambiguous definition locations, workspaceFolders-aware absolute resolution, workspace/symbol limits, and hierarchical document symbols.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~LspServerTests -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex exec review --ignore-user-config --ignore-rules --ephemeral --base origin/main

Full dotnet test CodeIndex.sln -p:UseSharedCompilation=false was also attempted and failed in the existing environment because RunPublishedTrimmedCli_SerializesQueryJsonAndErrorJson cannot publish without the required Microsoft.NETCore.App 10.0.8-servicing.26229.119 runtime; installed runtimes include 8.0.19, 9.0.5, 9.0.6, and 10.0.0.

Fixes #3427
Fixes #3428
Fixes #3537