Fix MCP search pagination stability

[Widthdom]

Summary

• add result_stable_at to MCP search responses so clients can detect index drift
• add opaque next_cursor/cursor pagination for MCP search pages
• document the MCP search pagination contract and add a changelog fragment
• adjust CI lint/build handling for existing trim/AOT diagnostics surfaced by this PR

Fixes #1462

Validation

• dotnet format whitespace CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• make lint
• dotnet build tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net8.0 --no-restore -p:UseSharedCompilation=false
• dotnet build tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net9.0 --no-restore -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~McpServerTests.ToolsCall_Search" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~McpServerTests" -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• dotnet run --project tools/CodeIndex.Changelog -- check