gh-144446: Fix thread safety of gi_frame, cr_frame and ag_frame in free-threading

[kumaraditya303]

• Issue: Data races with frame objects #144446

Fixes thread safety of reading gi_frame, cr_frame and ag_frame while the generator is running or finishing in another thread. Previously the reader could create a frame object for an interpreter frame that was concurrently being cleared, or two threads could each create a frame object for the same frame.

The frame object is now created within the generator's critical section, re-checking the frame state after acquiring it so a finished generator returns None instead of a frame for a cleared iframe. Clearing the generator's frame in gen_clear_frame() and clear_gen_frame() now also holds the critical section so it cannot race with the getter. frame->frame_obj is now set with a compare exchange as the running thread can create it without holding the generator's critical section (e.g. through sys._getframe()) — the losing thread discards its frame object and uses the existing one — and it is cleared with an atomic exchange and read with an acquire load.