fix: catch BaseException in cleanup loops to survive asyncio.CancelledError

[bkrabach]

Problem

asyncio.CancelledError is a BaseException subclass (since Python 3.9), so except Exception in kernel loop-based exception handling sites lets it escape, aborting remaining cleanup functions, hook handlers, and contribution collection. This causes resource leaks (open connections, file handles) and skipped lifecycle observers.

Inspired by #11 from @ramparte, who correctly identified this class of bug. This PR expands the fix to all affected sites with context-appropriate exception handling patterns, and adds comprehensive tests.

Approach

Different patterns per site semantics — one size does not fit all:

Site	Pattern	Rationale
collect_contributions	except CancelledError → break	Active work: honor cancellation, return partial results
Cleanup loops (coordinator.cleanup)	except BaseException → continue, re-raise fatal after	Cleanup MUST complete; re-raise KeyboardInterrupt/SystemExit after
session.cleanup	try/finally	Guarantee loader.cleanup() (sys.path restoration) always runs
session.execute	except BaseException → status tracking → re-raise	Bookkeeping before propagation
hooks.emit / emit_and_collect	except CancelledError → log, continue	All handlers should observe events (especially cleanup events like session:end)
cancellation.trigger_callbacks	except CancelledError → continue, re-raise fatal after	All callbacks must run; fatal exceptions honored after

Files Changed

• amplifier_core/coordinator.py — collect_contributions() + cleanup()
• amplifier_core/session.py — _safe_exception_str() + execute() + cleanup()
• amplifier_core/hooks.py — emit() + emit_and_collect()
• amplifier_core/cancellation.py — trigger_callbacks()
• tests/test_cancellation_resilience.py — 10 new tests covering all sites

Testing

• All 10 new cancellation resilience tests pass
• All 194 existing tests pass (1 pre-existing failure in test_event_taxonomy.py unrelated to this change)
• python_check (ruff + pyright) clean on all changed files

Closes

Supersedes #11