chore: align named child runs with the JS SDK

[vdusek]

Follow-up to #1149 and #1151 that aligns named child runs with the final version of apify/apify-sdk-js#759.

Changes:

• A different Actor, task or input under an existing run_name raises a ValueError only when the recorded run would be reused (reattached, returned as SUCCEEDED, or resurrected). When the recorded run FAILED or is gone, a new run starts with the new request, the old run moves to history, and the record takes the new checksum (agreed here).
• The client of a named run is stored only after the run is started or reused.
• The run_name docstrings describe the new rule.

No action on B4nan's comment about loading the child-run record lazily instead of in Actor init. JS kept the load in init(), because the synchronous Actor.childRuns getter needs the records, and Python already loads them the same way in __aenter__.

The docs in #1160 still say a name is bound to its first input. That will be fixed there.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.05%. Comparing base (3ed1a48) to head (a0f4ece).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1184   +/-   ##
=======================================
  Coverage   93.05%   93.05%           
=======================================
  Files          54       54           
  Lines        3671     3672    +1     
=======================================
+ Hits         3416     3417    +1     
  Misses        255      255           

Flag	Coverage Δ
e2e	31.72% <20.00%> (-0.01%)	⬇️
integration	51.68% <20.00%> (-0.02%)	⬇️
unit	89.78% <100.00%> (+<0.01%)	⬆️

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[Mantisus]

LGTM

[apify-approval-mirror]

Mirroring approval from @Mantisus.

[barjin]

lgtm, just a small nit regarding the docs (resolve at your discretion).

Thanks @vdusek !