fix(rtc): pass once() handlers only the arguments they take, and let off() remove them

[ANDROIDHASSAN]

Two ways a once() handler behaved differently from an on() one:

• emit() passes a handler only as many positional arguments as it takes (test_args). once() registered a *args wrapper instead of the callback, so the wrapper got every argument and passed them all on. A once handler that takes fewer than the event emits raised TypeError, which emit() re-raises:

  emitter.on("x", lambda a: ...)    # emit("x", 1, 2) calls it with 1
  emitter.once("x", lambda a: ...)  # emit("x", 1, 2) -> TypeError: <lambda>() takes 1 positional argument but 2 were given

• off(event, callback) looked up callback itself, so it could not remove a handler added with once(event, callback): the handler still ran on the next emit.

once() now registers a private _OnceHandler that keeps a reference to the callback (recognised with isinstance, so an on() handler is never mistaken for one). emit() trims the arguments to that callback's signature, read when the event fires as it is for on() handlers (so a functools.partial changed after registration is handled the same way). off() matches the reference the way a dict key is matched: identity, then hash, then ==, so an equal bound method removes it and an unrelated callback's __eq__ is never called. A callback with no signature, such as a builtin, still receives every argument, as before.

Tests in tests/rtc/test_emitter.py:

• test_once_passes_only_the_arguments_the_callback_takes: fails on main with the TypeError above.
• test_off_removes_a_once_handler: a function and a bound method added with once are removed by off; fails on main (the handler runs).
• test_once_runs_a_callback_without_a_signature: once(event, min) still runs and is removed.
• test_once_trims_to_the_callback_signature_at_emit_time, test_off_does_not_compare_unrelated_once_callbacks and test_an_on_handler_with_a_once_callback_attribute_keeps_its_signature: the cases from the review below.

tests/rtc and livekit-rtc/tests here, without the FFI library: the same 17 failures and 17 errors before and after, from the missing native library; 131 passed before, 137 now. ruff check, ruff format --check (0.15.4) pass; mypy (1.19.1) on livekit-rtc reports the same 4 errors in jupyter.py before and after.

Not changed: on() rejects async callbacks, but once() passes it the sync wrapper, so once(event, async_fn) is still accepted and its coroutine is never awaited.