Re-enable dill-based state fallback pickling on Python 3.15 once dill#753 is released

Follow-up to https://github.com/reflex-dev/reflex/pull/6930 (Python 3.15 support).

dill <= 0.4.1 cannot serialize functions on Python 3.15 (it uses code.co_lnotab, removed in 3.15): https://github.com/uqfoundation/dill/issues/753. As of today, 0.4.1 is still the latest release on PyPI, so #6930 marked the affected tests instead of fixing anything in Reflex:

• tests/units/test_state.py::test_fallback_pickle — @pytest.mark.xfail(sys.version_info >= (3, 15), raises=StateSerializationError)
• tests/integration/test_dynamic_components.py::test_dynamic_components — @pytest.mark.skipif(sys.version_info >= (3, 15) and bool(os.environ.get("REFLEX_REDIS_URL")))

Note this is not test-only fallout: on Python 3.15 with redis state, any state that needs the dill fallback (e.g. a Callable field, dynamic components) fails to serialize at runtime.

When a dill release fixing uqfoundation/dill#753 is out:

1. Bump dill in uv.lock (uv lock --upgrade-package dill).
2. Drop both markers and the accompanying TODO comments.
3. Confirm test_fallback_pickle and the redis-backed test_dynamic_components pass on 3.15.
4. Consider raising the declared minimum dill version so downstream users on 3.15 get the fix instead of a runtime serialization error.