fix(children): close iterators when React.Children.map callback throws

[mturac]

Summary

Fixes #37427 — React.Children.map (and forEach, count, toArray) manually iterates via iterator.next() but never calls iterator.return() when the map callback or recursive traversal throws.

Root cause: mapIntoArray in ReactChildren.js consumes the iterator in a while loop. If the user-supplied callback or the recursive mapIntoArray call throws, the function exits without signaling the iterator. Per the iterator protocol, a consumer that stops early must call return() so the producer can run cleanup (finally blocks in generators, resource release in custom iterables).

Fix: Wrap the iteration loop in try/finally. A tracking flag (iteratorDone) distinguishes normal completion from abrupt exit. The finally block calls return() only when the iterator did not run to completion, and swallows any error from return() itself to preserve the original error.

Changed files

• packages/react/src/ReactChildren.js — try/finally around iterator consumption in mapIntoArray
• packages/react/src/__tests__/ReactChildren-test.js — regression test: custom iterable with a return() spy, map callback throws on second element, verifies return() was called exactly once

Test plan

• New test: iterator return() spy called when map callback throws — passed
• Full ReactChildren-test.js suite — 43/43 passed

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