Reject malformed LSP request envelopes and symbol query types

[Widthdom]

Summary

• Require the exact JSON-RPC "2.0" envelope before LSP lifecycle, cancellation, dispatch, state, or database work.
• Require workspace/symbol params.query to be a string while preserving valid empty queries and the existing length limit.
• Add framed and staged fast-path regression coverage, bilingual integration/developer/testing documentation, and a changelog fragment.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false -m:1 -nr:false (0 warnings, 0 errors)
• Issue Reject malformed JSON-RPC envelopes and workspace/symbol query types in LSP #5231 tests in Debug and Release on net8.0 and net9.0 (2 passed per target/configuration)
• All LspServerTests in Debug (130 passed on net8.0; 130 passed on net9.0)
• Full Release net8.0 suite (12,151 passed, 9 skipped; one unrelated allocation probe failed under concurrent suite load and passed on isolated retry)
• Full Release net9.0 suite (11,665 passed, 433 skipped, 0 failed)
• dotnet run --project tools/CodeIndex.Changelog -- check
• Root and workspace index health checks
• Codex adversarial review: No blocking/actionable issues found. after addressing all first-pass findings

Documentation and changelog

• Updated the bilingual USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md sections.
• Added changelog.d/unreleased/5231.fixed.md.

Follow-up candidates

• None.

Fixes #5231