refactor(thread_pool): drop the -Wmaybe-uninitialized pragma; fix it properly (true-async/server#93)

[EdmondDantes]

Follow-up to #177. That PR silenced GCC -O2 -Wmaybe-uninitialized in thread_pool_worker_handler with a blanket #pragma GCC diagnostic ignored "-Wmaybe-uninitialized" over the whole function — a poor tool: it hides any future real uninitialized-variable bug there. This replaces it with proper, targeted fixes (no pragma, no Zend/ change):

• volatile on the five loop-locals that live across the loop's zend_try (state, snapshot, worker_coro, task_scope, body_bailed) — the C 7.13.2.1-correct fix for setjmp-live automatics.
• Split the two inner zend_try blocks (the bootloader call and the body await) into small guarded helpers (thread_pool_call_guarded, thread_pool_suspend_guarded). GCC's misfire on the zend_try macro's own __orig_bailout only happens across nested zend_try; with no nesting it's gone.

Result: zero -O2 warnings in the file, no suppression, no core change. thread_pool suite 78/79 (the one WARN is the pre-existing XFAIL-that-passes), behavior unchanged.

[codecov]

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