Task-safe Python region state: per-thread cache, TaskState refactor, and fast path optimizations

[cjdoris]

Motivation

• Make PythonCall region management task-safe by pinning task sessions to Julia threads and keeping a thread-local cached PyThreadState to avoid expensive lookups for nested regions.
• Ensure that @pyregion and @pyregionbreak can efficiently handle Python-originated calls (borrowed thread state) and nested regions without unnecessary synchronization.

Description

• Replace the old ThreadState/TaskState layout with a new ThreadState that holds a semaphore, a lazily-allocated tstate, and cached task/task_state, and extend TaskState with fields sem, attached, tid, and oldsticky to manage session lifecycle.
• Add helper functions current_task_state, set_task!, and clear_task! and update start_session!, enter_region, exit_region, enter_break, and exit_break to pin tasks, manage stickiness, acquire/release semaphores, and maintain the thread-local cache invariant.
• Optimize @pyregion macro to take a fast path when current_tstate() != C_NULL to avoid task- and thread-local lookups for Python-originated or nested regions, and update @pyregionbreak to use the cached current_task_state.
• Adjust semantics so that nested regions inside a break behave correctly, and ensure the cache is cleared before releasing the semaphore and restored after reacquiring it.
• Update tests: remove test/Region.jl and add expanded checks to test/C.jl under the new test item "Python regions and task-safe thread states" that validate cache ownership, nested region behavior, breaks, exception propagation, multi-threaded spawning, and callback behavior.

Testing

• Ran the updated unit tests in test/C.jl, including the new "Python regions and task-safe thread states" test item, which exercises @pyregion, @pyregionbreak, nested regions, and multi-threaded use; all tests passed.
• Existing Python info tests (python_executable_path, python_library_path, python_library_handle, python_version) were also executed and succeeded.

---

Codex Task