Fix race in event wait setup

[Aglcr7]

Closes #61

Summary

• Marks the wait Future as running before registering event callbacks in prepare_and_wait_for_event().
• Prevents an immediately delivered event from completing the future before set_running_or_notify_cancel() is called.
• Adds a regression test that simulates an event firing during registration without starting a real libmpv instance.

Fixes #61.

Verification

• python -m py_compile mpv.py tests/test_mpv.py
• git diff --check

Notes

• I could not run the focused unittest locally because this Windows environment does not have mpv-1.dll, mpv-2.dll, or libmpv-2.dll available for import mpv.