[codex] Bound MCP diagnostic echoes

[Widthdom]

Summary

• Bound MCP diagnostics for oversized enum-like scalar arguments, unknown argument names, unknown tool names, protocol versions, clientInfo fields, prompt inputs, and resource URIs.
• Added shared bounded display helpers and propagated truncation metadata into JSON-RPC errors, batch slots, telemetry, and audit records where applicable.
• Added focused MCP/Audit/Metrics tests for the new bounds, including adversarial-review follow-ups for metrics labels, batch slot metadata, and audit key collisions.

Validation

• dotnet format whitespace CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~McpServerTests|FullyQualifiedName~McpAuditLogTests|FullyQualifiedName~AuditLogSinkTests|FullyQualifiedName~MetricsSinkTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Changelog

• changelog.d/unreleased/3116.security.md
• changelog.d/unreleased/3117.security.md
• changelog.d/unreleased/3118.security.md
• changelog.d/unreleased/3119.security.md
• changelog.d/unreleased/3120.security.md
• changelog.d/unreleased/3121.security.md
• changelog.d/unreleased/3122.security.md

Notes

No target issue was excluded for an existing work-started comment. No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes are included.

Fixes #3116
Fixes #3117
Fixes #3118
Fixes #3119
Fixes #3120
Fixes #3121
Fixes #3122