feat: share the parent's charge budget with named child runs

[vdusek]

Stacked on #1154.

When the parent run was started with a max_total_charge_usd, its named child runs share that budget. A named Actor.start or Actor.call reserves a charge limit for its child from the part of the budget the parent hasn't charged or reserved for other children. Without an explicit max_total_charge_usd the child gets all of that part, and a higher value is lowered to it. A start in flight counts as reserved, so parallel starts can't overspend. When the budget is used up, a named start raises a RuntimeError.

The reservations live in the child run registry, so they survive a migration of the parent. ChargingManager subtracts them from the parent's own remaining budget, which keeps the whole tree of runs within the limit. When a child finishes, only its usage_total_usd stays reserved. A failed child that gets replaced keeps its charge counted, and a resurrected child gets a new limit that includes its own earlier reservation. Runs without a name aren't tracked and pass their limit through unchanged.

Two differences from the proposal in the issue, both based on apify-core:

• The platform gives every pay-per-event run a default limit (999,999,999 USD, capped at the account's remaining monthly usage). Splitting that would make the second parallel child fail, so the budget only applies when the run options say isMaxTotalChargeUsdSetByUser. That flag is read once from the parent run.
• usageTotalUsd keeps growing for about 3 minutes after a run finishes, so an early read under-reports the charge. The SDK re-reads a finished child until then and only stores the charge as final after that.

Closes #1139

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.61905% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.36%. Comparing base (5bbadc2) to head (472ac5d).

Files with missing lines	Patch %	Lines
src/apify/_child_runs.py	97.97%	2 Missing ⚠️
src/apify/_charging.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                          @@
##           feat/child-run-concurrency-limit    #1155      +/-   ##
====================================================================
+ Coverage                             93.29%   93.36%   +0.06%     
====================================================================
  Files                                    54       54              
  Lines                                  3804     3916     +112     
====================================================================
+ Hits                                   3549     3656     +107     
- Misses                                  255      260       +5     

Flag	Coverage Δ
e2e	32.78% <31.74%> (-0.08%)	⬇️
integration	51.63% <35.71%> (-0.42%)	⬇️
unit	90.29% <97.61%> (+0.15%)	⬆️

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