fix: saturate guild activity message counts

[vycdev2]

Summary

• keep guild-wide activity message counts at int.MaxValue instead of wrapping negative
• add a regression test for the saturation boundary
• document the fix in the changelog

Verification

• dotnet build — passed with 0 warnings and 0 errors
• dotnet test --no-build — passed: 608 tests; 3 culture-dependent tests skipped in globalization-invariant mode
• git diff --check — passed

Risk

• Low: the change only affects activity records after the guild counter reaches the integer limit.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.