Fix MCP issue handling and search tool metadata

[Widthdom]

Summary

• Accept the advertised MCP index.maxFileBytes argument.
• Add github_submission_reason and optional github_submission_error to MCP suggest_improvement responses.
• Shorten the MCP search tool description and keep detailed guidance in USER_GUIDE.md.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~McpServerTests.ToolsCall_IndexAllowsAdvertisedMaxFileBytesArgument -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~McpServerTests.SuggestImprovement_ValidInput_ReturnsSuccess -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~McpServerTests.ToolsList_SearchDescriptionStaysCompact -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

After merging current origin/main, dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~McpServerTests -p:UseSharedCompilation=false is blocked during build by existing Native AOT IL3050 analyzer errors. Tracked separately in #2786.

Docs And Changelog

• Updated USER_GUIDE.md for suggest_improvement GitHub submission reasons.
• Added changelog fragments:
  • changelog.d/unreleased/1460.fixed.md
  • changelog.d/unreleased/1640.changed.md
  • changelog.d/unreleased/2752.fixed.md

Follow-up

• dotnet test fails with Native AOT IL3050 analyzer errors #2786

Fixes #1460
Fixes #1640
Fixes #2752