gh-120321: fix thread safety of concurrently iterating over async generators

[kumaraditya303]

• Issue: SIGSEGV with generators in free-threaded build #120321

This PR fixes thread safety of async generators similar to #142599

[mpage]

LGTM!

[nascheme]

Some feedback from a review session using Claude Opus. There are some edge cases that would be good to fix:

• The claim on the async generator should only have one owner. proposed fix. This adds a new state "RUNNING" and renames the "ITER" state to "SUSPENDED". This seems like the most important one in terms of thread-safety.
• Use CAS when closing the awaitable on a finished frame. commit
• Close athrow awaitable when the operation completes. commit
• Validate the athrow() argument before claiming the generator. commit
• Document that a completed athrow() awaitable cannot be reused. commit

Full branch with commits on top of this PR branch. I'm not too familiar with the genobject.c code so the changes in that branch definitely need a second review before merging as-is.

I also found an additional thing but I'll make a new issue: logic related to ag_hooks_inited appears to be racey.