Summary

Bulbs and light strips can fade smoothly when their brightness or colour changes, instead of snapping straight to the new value. The Tapo app exposes this as "Smooth Transition". python-kasa cannot read or change it today, so the setting is missing from Home Assistant even though the device supports it. This PR adds it as a switch.

Under the hood, on_off_gradually reports three flags. on_state and off_state cover fading when the light is turned on and off, and are already exposed as the smooth_transition_on / smooth_transition_off number features. The third, change_state, appears nowhere in lighttransition.py: it is never read and never written, and set_enabled() only touches the other two.

• read change_state in _post_update_hook
• add a change_transition property and set_change_transition()
• register a smooth_transition_change switch feature when the device reports the flag

Verification

Confirmed on a physical L920 running firmware 1.4.4, on_off_gradually v4, by toggling the flag and watching the strip. With change_state cleared, a brightness jump from 10 to 100 snaps instantly. With it set, the same jump ramps. Turn-on and turn-off fades were unaffected in both cases, which is the expected split with on_state / off_state.

The flag persists across a full disconnect, so it is stored device state rather than a per-session toggle.

Notes

• Only newer firmware reports change_state. Exactly one fixture in the repo has it, L530EA(EU)_1.0_1.4.2, and 26 other on_off_gradually fixtures do not, so the feature is registered conditionally and the test asserts both paths.
• The conditional checks the parsed value rather than self.data. _initialize_features runs after _post_update_hook and must not raise for devices whose get_on_off_gradually_info query failed. Reading self.data there breaks fixture generation for several plugs, including P135 and KS225.
• tests/fakeprotocol_smart.py needed a matching branch in _set_on_off_gradually_info, which previously applied on_state and off_state but silently dropped change_state.
• The feature is a switch rather than a number because the device reports only enable for this flag, with no duration alongside it.

Test plan

• uv run pytest tests/smart/modules/test_lighttransition.py - 66 tests, covering both the supported and unsupported paths
• uv run pytest - full suite passes
• uv run pre-commit run -a - clean