fix(aws): avoid cancelling the active Bedrock recycle task

[nightcityblade]

Fixes #5879

Summary

• avoid cancelling the currently running recycle timer while it is bringing up the replacement Bedrock session
• keep cancelling any older recycle timer task before arming a new one
• add regression tests covering both self-rearm and previous-timer replacement

Testing

• uv run --extra realtime pytest -q tests/test_aws_realtime_session_recycle.py
• ruff check livekit-plugins/livekit-plugins-aws/livekit/plugins/aws/experimental/realtime/realtime_model.py tests/test_aws_realtime_session_recycle.py

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[nightcityblade]

Pushed 44d2a82 to address the CI failure. The new tests/test_aws_realtime_session_recycle.py module now declares pytestmark = pytest.mark.unit, which matches the repo's required test categorization and should unblock the unit-tests job.

Validation: uv run ruff check tests/test_aws_realtime_session_recycle.py

I wasn't able to run that test module end-to-end locally on macOS because collection hits a missing aws_sdk_bedrock_runtime import in this environment, but the failing CI signal was the missing category marker and that is now fixed.