Fix LSP textDocument URI length cap

[Widthdom]

Summary

• Reject oversized LSP textDocument.uri values before URI parsing and path normalization, using the MCP resource URI cap.
• Add an LSP regression test for the bounded -32602 error response.
• Document the LSP URI cap and add the unreleased changelog fragment.

Validation

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

Fixes #3129