fix(core): Prevent lost wakeups in batch processors (#5756)

* fix(core): Prevent lost wakeups in batch processors

Coordinate log and metric flush scheduling through an atomic state transition. This ensures events added while a flush completes always have a pending task instead of relying on the running Future's completion state.

Fixes #5739

Co-Authored-By: Claude <noreply@anthropic.com>

* changelog

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Co-authored-by: Claude <noreply@anthropic.com>