feat: add MopDryerTrait for controlling the dock mop dryer

[dschuld]

Summary

A switch trait for starting and stopping a mop drying cycle on docks that support
drying, via APP_SET_DRYER_STATUS.

dry_status is already readable. It was annotated with dps metadata in #740 and is
consumed by Home Assistant through is_field_supported(StatusV2, StatusField.DRY_STATUS). But there is no trait behind the write side, so callers have to fall back to a raw command.

mop_dryer = device.v1_properties.mop_dryer
if mop_dryer is not None:
    await mop_dryer.enable()   # start drying
    mop_dryer.is_on            # True
    await mop_dryer.disable()  # stop

The trait is created in the special-case block in discover_features() alongside
WashTowelModeTrait and ObstaclePhotoTrait, since it takes a constructor argument.
It is gated on dock_features.is_dryable.

Background

#262 asked for this in Dec 2024 and was closed by the reporter with:

solved with app_set_dryer_status with param status:1

APP_SET_DRYER_SETTING configures the dry duration; APP_SET_DRYER_STATUS starts and
stops the cycle. That finding never made it into the library. I've confirmed the
command works against my own dock.

Open Issues / Design Choices

1. Where the state comes from. Unlike the other switch traits, the dryer has no query
command of its own. If a cycle is running is reported as dry_status on StatusV2.
So this trait holds the status trait, reads is_on from it, and overrides refresh() to
delegate, leaving V1TraitMixin's command/converter unused.

That is a deliberate deviation from the pattern documented in the module docstring
("subclass V1TraitMixin and a RoborockBase dataclass… you must define a command
class variable"). I would like to point this out here rather than have it found in review.
The alternative is a self-contained trait backed by APP_GET_DRYER_SETTING with a new
DryerSetting dataclass, which would also expose the dry-duration setting. I didn't go
that way because I have no sample of that response and didn't want to guess at the wire
format. I am happy to capture it from my dock and rework this if this approach is preferred.

Testing

50 new tests in tests/devices/traits/v1/test_mop_dryer.py:

• trait is created for all dock types where is_dryable is true, and absent for all where
  it is false (parametrized over the full RoborockDockTypeCode enum)
• is_on mapping for dry_status of None / 0 / 1
• enable() / disable() send APP_SET_DRYER_STATUS with the right params and apply the
  optimistic update
• refresh() delegates to the status trait

100% line and branch coverage on the new module.

AI assistance disclosure

This contribution was prepared with Claude Code assistance. I reviewed the submitted changes and test results and take responsibility for the contribution.

Closes #905