…s to DataQueue (apache#62947)

The DataQueue implementation used parallel vectors (one per child queue
—
lock, blocks, counters, etc.) scattered across the class, making it hard
to
reason about which lock protects which field. The old INJECT_MOCK_SLEEP
pattern injected test randomness through a macro wrapping
std::lock_guard,
but this was fragile and didn't compose with Clang's -Wthread-safety
static
analysis. Several methods (e.g., set_source_block) performed lock-free
  checks followed by locked re-checks, risking subtle races.

Root cause: per-child state was not encapsulated, lock/field
relationships
  were implicit, and no static analysis guarded them.
This PR:

- Introduces a SubQueue struct that groups all per-child state (queue
lock
+ blocks, free lock + free blocks, counters, sink dependency pointer)
with
explicit GUARDED_BY annotations.
thread-safety macros, an AnnotatedMutex wrapper, and a LockGuard that
replaces std::lock_guard. In BE_TEST builds, LockGuard injects random
sleep
before lock acquisition and after release to exercise concurrent code
paths — replacing the old INJECT_MOCK_SLEEP macro.
  - Enables -Wthread-safety in Clang builds.
  - Moves dependency notifications (set_ready, block, set_always_ready)
outside the queue lock in try_pop/try_push/clear_blocks to avoid nested
lock ordering issues.
- Fixes set_source_block to always hold _source_lock when reading
_source_dependency, eliminating the lock-free pre-check.
- Adds 20+ new unit tests covering SubQueue methods and DataQueue edge
  cases (empty pop, push-after-finished, capacity blocking, finish
idempotency, clear_blocks, low-memory mode, terminate, free-block reuse,
child_idx routing).

(cherry picked from commit efd7067)