Fix MCP diagnostic sanitization and protocol bounds

[Widthdom]

Summary

• Sanitized MCP tool, worker, and batch diagnostics so raw exception details are not echoed on the affected paths.
• Coarsened HTTP MCP auth failure telemetry and normalized stdio/HTTP bearer token shape validation with matching docs.
• Added bounded line reads for MCP stdio and worker protocols while preserving large-file protocol opt-ins.

Issues

Fixes #3370
Fixes #3425
Fixes #3469
Fixes #3505
Fixes #3506

Validation

• dotnet build -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Issue3370|FullyQualifiedName~Issue3425|FullyQualifiedName~Issue3469|FullyQualifiedName~Issue3505|FullyQualifiedName~Issue3506|FullyQualifiedName~Issue3429|FullyQualifiedName~Issue3432|FullyQualifiedName~Issue3461|FullyQualifiedName~RunBatch_TooDeepJsonLine_ReturnsUsageError_Issue3022|FullyQualifiedName~StartInfo" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: final targeted re-review reported no blocking findings.

Notes

• All requested issues had no third-party work-start comments when checked; none were excluded.
• Full-suite failures observed earlier matched existing unrelated issues Trimmed publish test fails when ILLink 10.0.8 requires unavailable .NET 10.0.8 runtime #3571/GitHelper timeout test now fails during commit validation #3572/Published trimmed CLI test depends on live restore/runtime environment #3573, so no new issue was opened.
• Updated USER_GUIDE.md and DEVELOPER_GUIDE.md. Did not modify AGENT.md, CLAUDE.md, or AGENT_GUIDE.md.