feat: add Actor.child_runs() to list named child runs

[vdusek]

Stacked on #1149. Review and merge that one first.

Actor.child_runs() returns the named child runs of the current Actor run, keyed by name. Each ChildRunInfo has the recorded actor_id and run_id, the run as the API returns it at call time (None once the platform no longer has it), and previous_run_ids with the earlier runs under that name that failed and were replaced. Runs started without a name aren't tracked, so they aren't listed.

The runs are fetched concurrently on each call, one GET per name. Nothing is cached, so the result is a snapshot. There's no status callback, which the issue leaves out of scope. A caller who wants progress reporting can poll child_runs().

The "Named child runs" docs section gets a "Listing child runs" subsection with an example.

Closes #1129

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.13%. Comparing base (82366a3) to head (08fe7c0).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           feat/named-child-runs    #1151      +/-   ##
=========================================================
+ Coverage                  93.09%   93.13%   +0.03%     
=========================================================
  Files                         54       54              
  Lines                       3651     3672      +21     
=========================================================
+ Hits                        3399     3420      +21     
  Misses                       252      252              

Flag	Coverage Δ
e2e	33.08% <82.60%> (+0.27%)	⬆️
integration	52.91% <82.60%> (+0.16%)	⬆️
unit	89.84% <100.00%> (+0.05%)	⬆️

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