…ergeTree

The `DataValidationTasks` struct stored a `std::unique_lock<std::mutex>`
holding a lock on `BackgroundSchedulePoolTaskInfo::exec_mutex`. This caused
a ThreadSanitizer warning because:

1. Thread A acquires the lock via `pausePartsCheck()` -> `getExecLock()`
2. `DataValidationTasks` is stored in a `shared_ptr` and copied to worker
   threads during parallel CHECK TABLE execution (via `TableCheckTask`)
3. When the last reference is destroyed (potentially on Thread B), the
   `unique_lock` destructor tries to unlock the mutex
4. pthread mutexes must be unlocked by the same thread that locked them,
   causing the TSan error "unlock of an unlocked mutex (or by a wrong thread)"

The fix replaces `pausePartsCheck()` (which returned a `unique_lock`) with
`temporaryPause()` which returns a new `TemporaryPause` RAII guard. This
guard uses `deactivate()` to pause the background task and `activateAndSchedule()`
in its destructor to resume it. Both operations are thread-safe and can be
called from any thread, eliminating the thread-affinity issue.

Changes:
- Add `getTaskInfoPtr()` to `BackgroundSchedulePoolTaskHolder`
- Add `TemporaryPause` class to `ReplicatedMergeTreePartCheckThread`
- Remove `pausePartsCheck()` and replace all usages with `temporaryPause()`
- Update `DataValidationTasks` to use `TemporaryPause` instead of `unique_lock`

Fixes #87916

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>