feat: abort named child runs when the parent is gracefully aborted

[vdusek]

Stacked on #1151.

Adds abort_with_parent to Actor.start and Actor.call. It's off by default and needs name. The flag is stored with the name in the child run registry, so a parent that migrated or was resurrected still knows which children to abort. The latest call under a name sets the value.

The SDK registers its own ABORTING listener on ApifyEventManager when the Actor context starts. It's kept apart from user listeners, so off() without a listener doesn't remove it. On a graceful abort, it gracefully aborts every flagged child that's still READY or RUNNING, all in parallel. If one abort fails, the error is logged and the others go ahead. Each child is aborted with the client it was started with in this process, which matters when a custom token was passed. A child from before a migration uses the default client. ChildRunInfo now exposes the flag too.

A hard abort, a timeout or an OOM kill still leaves the children running. Covering that needs a parent/child link on the platform.

Closes #1128

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.21%. Comparing base (08fe7c0) to head (9554a97).

Additional details and impacted files

@@                   Coverage Diff                   @@
##           feat/child-runs-api    #1153      +/-   ##
=======================================================
+ Coverage                93.13%   93.21%   +0.07%     
=======================================================
  Files                       54       54              
  Lines                     3672     3728      +56     
=======================================================
+ Hits                      3420     3475      +55     
- Misses                     252      253       +1     

Flag	Coverage Δ
e2e	33.02% <26.98%> (-0.07%)	⬇️
integration	52.60% <31.74%> (-0.32%)	⬇️
unit	89.99% <100.00%> (+0.15%)	⬆️

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