Add processor graceful shutdown

[karenychen]

Summary

• Add Processor.Close(ctx) to stop receiving and cancel in-flight handlers during shutdown.
• Track in-flight messages so close can abandon the messages currently held by the processor.
• Abandon tracked messages concurrently as best effort while respecting caller context cancellation.
• Keep failed abandon attempts tracked so a later close can retry them.

Testing

• go test -run TestInFlightMessages .
• go test -race -run 'TestInFlightMessages|TestProcessor' .\n- go test ./...\n- git diff --check

[karenychen]

Code review

No issues found. Checked for bugs and AGENTS.md compliance.