fix: honor bot avatar job cancellation

[vycdev2]

Summary

• pass the Quartz cancellation token through the bot-avatar settings lookup, Discord avatar request, and persistence
• propagate requested cancellation instead of swallowing it as an avatar-update error
• add a regression test through the scheduled job entry point

Verification

• dotnet build --no-restore — passed: 0 errors (1 existing SQLite package vulnerability warning)
• dotnet test --no-restore --filter 'FullyQualifiedName~BotAvatarJobTests' --logger 'console;verbosity=minimal' — passed: 1/1
• dotnet test --no-restore --logger 'console;verbosity=minimal' — failed: unrelated LogsWriterServiceTests.ExecuteAsync_RetriesBatchAfterTransientPersistenceFailure timed out in the parallel run; 428 passed, 1 failed, 1 skipped
• dotnet test --no-restore --filter 'FullyQualifiedName~LogsWriterServiceTests.ExecuteAsync_RetriesBatchAfterTransientPersistenceFailure' --logger 'console;verbosity=minimal' — passed: 1/1 in isolation
• dotnet test --no-restore --logger 'console;verbosity=minimal' -- xunit.parallelizeTestCollections=false — passed: 429, skipped: 1, failed: 0
• git diff --check upstream/develop...HEAD — passed

Risk

• Low — normal avatar selection and update behavior is unchanged; the job now stops promptly when Quartz requests cancellation.

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