feat: reattach named child runs after migration or resurrection

[vdusek]

Actor.start and Actor.call accept an optional name. The SDK writes the name and run ID to the default key-value store (APIFY_CHILD_RUNS) right after the child starts. After a migration or resurrection of the parent, the same call finds the recorded run and reuses it:

• READY / RUNNING: reattaches to it
• SUCCEEDED: returns it as is
• ABORTED / TIMED-OUT: resurrects it (an ABORTING / TIMING-OUT run is waited for first)
• FAILED, or the run no longer exists: starts a new run and keeps the old ID in previousRunIds

A named call on a reattached or resurrected run streams only new log lines. Reusing a name for a different Actor raises ValueError, and concurrent calls under one name in the same process share a single run.

A hard kill between the platform starting the child and the registry write can still orphan the child. Closing that gap needs an idempotency key on the run-start endpoint.

Closes #1127

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.09%. Comparing base (34a12d5) to head (82366a3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1149      +/-   ##
==========================================
+ Coverage   92.89%   93.09%   +0.20%     
==========================================
  Files          53       54       +1     
  Lines        3560     3651      +91     
==========================================
+ Hits         3307     3399      +92     
+ Misses        253      252       -1     

Flag	Coverage Δ
e2e	32.81% <40.42%> (+0.20%)	⬆️
integration	52.75% <40.42%> (-0.29%)	⬇️
unit	89.78% <100.00%> (+0.26%)	⬆️

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