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 run_id, the run as the API returns it at call time, and history: the earlier runs under that name that failed or went missing and were replaced, as ChildRunSnapshot entries. run is None when the platform no longer has the run or the fetch failed. A failed fetch also logs a warning. Runs started without a run_name aren't tracked, so they aren't listed.

Every call fetches the runs again, at most 10 at a time. Nothing is cached, so the result is a snapshot. A run started with a custom token is fetched with that token. After a migration the token is gone and the default client is used. The issue leaves a status callback out of scope, so there isn't one. A caller who wants progress reporting can poll child_runs().

Docs are in #1160.

Closes: #1129

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (c9856b0) to head (3892bb1).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           feat/named-child-runs    #1151      +/-   ##
=========================================================
+ Coverage                  93.17%   93.20%   +0.03%     
=========================================================
  Files                         54       54              
  Lines                       3690     3723      +33     
=========================================================
+ Hits                        3438     3470      +32     
- Misses                       252      253       +1     

Flag	Coverage Δ
e2e	33.25% <61.76%> (?)
integration	52.78% <61.76%> (+0.04%)	⬆️
unit	89.98% <100.00%> (+0.08%)	⬆️

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[vdusek]

TODO for child_runs():

• Cap the number of concurrent run fetches (e.g. 10-20). It's an unbounded asyncio.gather now, so hundreds of named children mean hundreds of simultaneous GETs.
• Don't let one failed fetch fail the whole call. Return that entry with run=None and log a warning, as counting toward the concurrency limit already does (return_exceptions=True).

✍️ Drafted by Claude Code

[Mantisus]

Actor.start, Actor.call and Actor.call_task accept a token, but child_runs() fetches every run with the default client. Is a run started with a different token supposed to be supported here?