refactor: introduce typed SceneAccessor and TimerAccessor

[Faerkeren]

Closes #76

Issue validity

Valid and actionable. The issue correctly identifies that:

• DomainSpec.operations (dict[str, Callable[..., Any]]) and dynamic setattr binding make collection-level domain operations invisible to type checkers.
• Both scene.py and timer.py worked around this by accessing the private accessor._factory with # type: ignore[assignment], bypassing encapsulation.
• The project ships py.typed and runs mypy in strict mode, so this is a genuine gap.

Fix

core/plugins.py

• Added accessor_cls: type[DomainAccessor[Any]] | None = None field to DomainSpec. Domains with collection-level operations declare their typed accessor subclass here instead of populating operations.
• Added DomainAccessor.factory as a public property returning the EntityFactoryProtocol. Subclasses use this to reach factory.services and factory.state without touching _factory.

api.py

• HAClient.__init__ now instantiates spec.accessor_cls when set, falling back to the base DomainAccessor for domains that don't need collection-level operations.

domains/scene.py

• Replaced free functions _create / _apply + operations={...} with a SceneAccessor(DomainAccessor[Scene]) subclass exposing typed async def create(...) -> Scene and async def apply(...) -> None methods.
• DomainSpec now uses accessor_cls=SceneAccessor.

domains/timer.py

• Replaced free function _create + operations={...} with a TimerAccessor(DomainAccessor[Timer]) subclass exposing a typed async def create(...) -> Timer method.
• DomainSpec now uses accessor_cls=TimerAccessor.
• All # type: ignore[assignment] comments removed.

Tests added (tests/test_plugins.py)

Test	What it verifies
test_domain_accessor_factory_property	DomainAccessor.factory returns an EntityFactory instance
test_scene_accessor_is_typed_subclass	ha.scene is a SceneAccessor with callable create and apply
test_timer_accessor_is_typed_subclass	ha.timer is a TimerAccessor with a callable create
test_accessor_cls_in_spec_is_used	Custom accessor_cls on a spec is correctly instantiated
test_accessor_cls_none_falls_back_to_base	Specs without accessor_cls get the base DomainAccessor

Validation

• pytest tests/ --cov=haclient --cov-fail-under=95: 316 passed, coverage 97.11%
• ruff check src tests: All checks passed
• ruff format --check src tests: 57 files already formatted
• mypy src: Success: no issues found in 38 source files