Enforce suggestion store read limits

[Widthdom]

Summary

• Enforce read-side JSON depth and record-count limits for SuggestionStore full, filtered, and paged reads.
• Treat over-deep or oversized suggestion stores like corrupt stores: preserve a .bak and return an empty result.
• Add regression coverage and a security changelog fragment for Issue SuggestionStore JSON reads should cap depth and record counts #3062.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~SuggestionStoreTests -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes
• git diff --check origin/main..HEAD
• Codex adversarial review: round 1 found a paged-read cap bypass; fixed and re-reviewed. Round 2 found no blocking/actionable issues.

Notes

• Changelog fragment: changelog.d/unreleased/3062.security.md
• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.
• Full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted but stopped after more than 19 minutes without a final summary.

Fixes #3062