Harden A2A task lifecycle

[kutterdred]

Summary

• add explicit Agent Zero A2A worker lifecycle logging around context.communicate and task.result
• wrap task.result with a bounded timeout and mark timed-out tasks failed
• return sanitized failure messages in task history so tasks/get can retrieve the failure state
• add focused regression coverage for timeout and exception paths

Validation

• /opt/venv-a0/bin/python -m py_compile helpers/fasta2a_server.py
• /opt/venv-a0/bin/python -m pytest tests/test_fasta2a_server_worker.py -q
• python tools/regression_harness.py --allow-known-failures in TEMERITY_CORE