Fix LSP malformed JSON and lifecycle exit handling

[Widthdom]

Summary

• Handle malformed LSP JSON-RPC payloads inside the message handler and return a parse-error response instead of letting the server loop terminate.
• Stop the stdio LSP loop on exit notifications, preserve successful shutdown -> exit termination, and return a non-zero process code when exit arrives before shutdown.
• Add focused LSP regression coverage and bilingual changelog fragments.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~LspServerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• changelog.d/unreleased/2829.fixed.md
• changelog.d/unreleased/2830.fixed.md

Follow-up candidates

None.

Fixes #2829
Fixes #2830