Cyber: prevent queued timer callback use-after-free

[timothyanderson096-ocdealcheck]

Summary

• replace the queued raw callback pointer with a weak TimerTask reference
• discard queued work when Timer::Stop() has already destroyed the task
• preserve the canonical callback object across periodic firings
• add regressions for stop-before-dequeue lifetime and mutable callback state

Rationale

Capturing a strong task reference would keep a stopped timer alive, while copying the callback would change stateful periodic-callback semantics. Locking a weak task reference at execution time avoids the reported use-after-free without either behaviour change.

Testing

• added TimerConcurrencyTest.stop_before_queued_callback_runs
• added TimerTest.periodic_callback_preserves_state
• standalone ASan reproducer: the original raw-pointer path reports heap-use-after-free; the weak-reference path completes without invoking the stopped callback
• git diff --check

The Apollo Bazel target could not be run locally because the Apollo development container/toolchain was unavailable. Repository CI will provide the project-level build and test result.

Fixes #15810

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