Implement correctness assertions for test_ema in Adam optimizer (#22979)

- Updated test_ema in keras/src/optimizers/adam_test.py with assertions for initialization, manual updates, and model.fit() integration.
- Added test_ema_overwrite_frequency to verify periodic model weight overwriting with EMA values.
- Verified changes with ruff and py_compile.

Co-authored-by: andersendsa <199610634+andersendsa@users.noreply.github.com>