fix!: honor explicit exit codes and control-flow signals in Actor exit

[vdusek]

Previously Actor.__aexit__ treated any BaseException as "the user function threw": it logged "Actor failed with an exception" and forced the exit code to 91 (EXIT_CODE_ERROR_USER_FUNCTION_THREW), ignoring any explicitly-requested exit code and mishandling control-flow signals.

Consequences:

• Actor.fail(exit_code=42, exception=e) silently exited 91, contradicting the fail() docstring and the error-handling guide example (which maps ValueError → 10).
• sys.exit(5) inside the Actor block exited 91 instead of 5.
• KeyboardInterrupt (Ctrl+C) became a logged failure + exit 91 instead of interrupt semantics.
• asyncio.CancelledError was consumed by sys.exit(91), breaking the cancellation contract.

Now __aexit__ dispatches on the exception type:

• a regular Exception still falls back to 91, but only when no exit code was set explicitly (so fail(exit_code=X, exception=e) honors X);
• SystemExit keeps its own code;
• every other BaseException (Ctrl+C, task cancellation, ...) is re-raised untouched after cleanup.

Added regression tests covering all four cases.

BREAKING CHANGE: Actor exit codes and failure logging are now accurate to the exception type. fail(exception=e) exits with the requested exit_code (default 1) instead of always 91; sys.exit(n) inside the Actor block exits n instead of 91; KeyboardInterrupt and asyncio.CancelledError propagate instead of becoming a logged failure with exit 91; and "Actor failed with an exception" is no longer logged for those control-flow cases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.71%. Comparing base (ed1d8eb) to head (7d38e9b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1048      +/-   ##
==========================================
- Coverage   91.75%   91.71%   -0.05%     
==========================================
  Files          50       50              
  Lines        3215     3222       +7     
==========================================
+ Hits         2950     2955       +5     
- Misses        265      267       +2     

Flag	Coverage Δ
e2e	35.56% <0.00%> (-0.08%)	⬇️
integration	57.35% <44.44%> (-0.04%)	⬇️
unit	83.05% <100.00%> (-0.03%)	⬇️

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