fix(core): Store child span timeout handle in _childSpanTimeoutID - #23429

_restartChildSpanTimeout stored its timer handle in _idleTimeoutID instead of _childSpanTimeoutID, so
_childSpanTimeoutID was never assigned and _cancelChildSpanTimeout() was dead at both of its call sites.

Root cause

On the idleSpanEnableAutoFinish path, _restartIdleTimeout() arms the idle timer and the immediately
following _restartChildSpanTimeout() overwrites its handle without clearing it, orphaning an armed idle timer
that nothing can cancel. When the in-flight children finish, _popActivity's _cancelIdleTimeout() cancels the
child timer instead, and the orphan later fires and ends the idle span at signalTime + idleTimeout rather than
lastChildEnd + idleTimeout. Child spans starting in that window are dropped from the transaction.

The other caller, _pushActivity, is unaffected because _cancelIdleTimeout() already runs immediately before
it — which is why no existing test covered this.

childSpanTimeout itself kept working by accident via _cancelIdleTimeout, so this does not change its
behaviour; it only makes the two timers independent, as the code already intended.

Fixes #23404

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