[codex] Bound MCP progress tokens

[Widthdom]

Summary

• Bounds _meta.progressToken before cloning or echoing it in MCP progress notifications.
• Accepts only small scalar/object tokens and ignores unsupported or oversized shapes.
• Updates index / backfill_fold tool descriptions and adds regression coverage for accepted and rejected tokens.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter FullyQualifiedName~McpServerTests.RunAsync_IndexWith
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter FullyQualifiedName~HttpMcpTransportTests.HttpTransport_IndexWithProgressToken
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• codex exec review --base origin/main --dangerously-bypass-approvals-and-sandbox --dangerously-bypass-hook-trust

Documentation / Changelog

• changelog.d/unreleased/3103.security.md
• MCP tool descriptions for index and backfill_fold were updated.

Follow-ups

• None.

Fixes #3103