feat: add a concurrency limit for named child runs

[vdusek]

Stacked on #1153.

Adds Actor.set_child_run_limits(max_concurrent_runs=...). While that many named child runs are READY, RUNNING, ABORTING or TIMING-OUT, a named Actor.start or Actor.call that would start or resurrect a run waits for one of them to finish. Reattaching to a recorded run never waits, and runs without a name aren't counted.

The count comes from the child run registry, so children started before a migration hold their slots too. A child's status is fetched again before counting once it's more than 10 seconds old, so a finished run nobody waited on frees its slot. Actor.call frees the slot as soon as its run finishes. A start holds its slot until the run is recorded, which keeps concurrent starts under different names within the limit.

The limit lives in memory, so the Actor sets it again after a migration. Once the parent receives ABORTING, a start still waiting for a slot raises a RuntimeError without starting a run. Waiting further would block the abort of the other children.

Closes #1138

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.29%. Comparing base (9554a97) to head (5bbadc2).

Files with missing lines	Patch %	Lines
src/apify/_child_runs.py	97.36%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           feat/abort-child-runs-with-parent    #1154      +/-   ##
=====================================================================
+ Coverage                              93.21%   93.29%   +0.08%     
=====================================================================
  Files                                     54       54              
  Lines                                   3728     3804      +76     
=====================================================================
+ Hits                                    3475     3549      +74     
- Misses                                   253      255       +2     

Flag	Coverage Δ
e2e	32.86% <23.75%> (-0.17%)	⬇️
integration	52.05% <23.75%> (-0.56%)	⬇️
unit	90.14% <97.50%> (+0.14%)	⬆️

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