[aevnsvi]

Closes #8344

Every DownloaderFactory registered its own bound _session_cleanup with atexit, and the atexit registry holds it until the process exits. The content app builds a factory for every file it streams from a remote, so each streamed file kept a factory, its remote and its closed aiohttp session in memory for the life of the worker. Details are in #8344.

What changes

• The factories are tracked in a module-level weakref.WeakSet, and one atexit hook closes the sessions that are still open at exit. A factory that is no longer used can now be freed.
• _session_cleanup skips a session its owner has already closed, so the exit hook no longer runs an event loop for it (on Python 3.12+ that failed with "There is no current event loop" for every factory ever created, see ClientSession teardown runs after event loop closes #6442).
• No API change; the content handler keeps closing the session in its finally block as before.

Testing

• Two unit tests in test_downloader_factory.py:

  • a factory whose session was closed is freed after gc.collect();
  • the exit hook closes a session that is still open, and a second run does nothing.

  Both fail without the change.

• Reproduction: 300 factories built and closed the way the content handler does it; all 300 are still alive after gc.collect() on main, none with the change.

• The pulpcore unit suite gives the same results with and without the change.

Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added