feat: add QV 35A dock type code (21) to RoborockDockTypeCode

[epinethrone]

Summary

Adds dock type 21 — the wash/fill/dry dock shipped with the Roborock QV 35A (roborock.vacuum.a168) — to RoborockDockTypeCode and to WASH_N_FILL_DOCK_TYPES.

This mirrors #829 (which added the Qrevo S5V, 22).

Fixes #864.

Why

The QV 35A's dock reports dock_type: 21, which had no enum member, so RoborockDockTypeCode(21) fell back to unknown. That made is_wash_n_fill_dock() return False, which skips the wash_towel_mode trait at setup (Trait 'wash_towel_mode' not supported, skipping). Downstream in Home Assistant, the clean-water-box, dirty-water-box and strainer entities are gated on wash_towel_mode is not None, so they were permanently unavailable — even though the device reports the underlying data (dss for the tank statuses, strainer_work_times in the consumables payload).

Verified on a real QV 35A (firmware 02.36.38): after adding the dock type, is_wash_n_fill_dock is True and the trait is set up as expected. Clean-fluid delivery is not supported on this dock (is_clean_fluid_delivery_supported = False), so the clean_fluid_empty entity correctly remains absent.

Changes

• roborock/data/v1/v1_code_mappings.py: add qv_35a_dock = 21.
• roborock/device_features.py: add RoborockDockTypeCode.qv_35a_dock to WASH_N_FILL_DOCK_TYPES.
• Tests: a dedicated test_qv_35a_dock_type, plus the new dock type added to the wash_towel_mode / dust_collection_mode / smart_wash_params trait parametrizations — same coverage feat: add Qrevo S5V dock type code (22) to RoborockDockTypeCode #829 added.

Testing

pytest tests/data/ tests/devices/traits/v1/test_wash_towel_mode.py \
       tests/devices/traits/v1/test_dust_collection_mode.py \
       tests/devices/traits/v1/test_smart_wash_params.py \
       tests/test_supported_features.py tests/devices/traits/v1/test_device_features.py

All pass (snapshots unchanged).

[Lash-L]

Thank you for this PR! This has been something on my TODO list for a while now as we handle dock types a little poorly and rigidly. Inspired by this PR, I have actually made a PR that will solve a bit more of the problem at once and add a number of new dock types:

#867

I'm going to close out this PR as mine will handle a bit more, but I sincerely appreciate you making this PR!