Fix JobGroup state aggregation

[fallintoplace]

What changed

• add explicit JobGroup state aggregation instead of returning only self.states[0]
• make cleanup() wait until every tracked handle is terminal before running executor cleanup
• update JobGroup tests to cover mixed multi-handle states instead of only single-handle behavior

Why

JobGroup.state previously returned only the first handle's state, which let mixed groups report the wrong status. That leaked into status() and cleanup() behavior.

This fixes the main bad cases:

• [SUCCEEDED, FAILED] now reports FAILED
• [RUNNING, FAILED] no longer hides the failure
• [FAILED, RUNNING] still reports the failure, but cleanup() no longer runs until all handles are terminal

Aggregation rules

• FAILED if any handle failed
• RUNNING if any handle is still running and none failed
• PENDING or SUBMITTED if work is still queued and nothing is running or failed
• SUCCEEDED only if all handles succeeded
• CANCELLED when all handles are terminal and at least one was cancelled, with no failures
• UNKNOWN otherwise

Checks

• uv run -- pytest test/run/test_job.py
• uv run --group lint -- ruff check nemo_run/run/job.py test/run/test_job.py