fix: latest_available_function does not reliably support plugin datasets

[turban]

Problem

sync.availability.latest_available_function accepts a dotted-path string, implying plugin support. However, the resolution in sync_engine._get_dynamic_function uses plain importlib.import_module without ensuring plugins_dir is on sys.path first.

Plugin availability functions only resolve if the dataset registry happened to run earlier in the same process (which adds plugins_dir to sys.path as a side effect). This is an accidental dependency on call ordering, not a guarantee.

The other three dotted-path extension points — ingestion.function, transforms:, and execution_function — all explicitly handle plugins_dir before resolving. This one does not.

Proposed resolution

Remove the implicit plugin support and restrict latest_available_function to built-in functions in climate_api/providers/availability.py. Availability logic is tightly coupled to provider behaviour (lag windows, release schedules) and belongs in core code, not plugins.

If a new provider cadence is needed, the right fix is a new built-in function or a new parameter on an existing one — not a plugin hook.

Acceptance criteria

• Document that latest_available_function only supports built-in dotted paths
• Optionally: validate at registry load time that the function resolves to a known built-in module, and raise a clear error if a plugin path is supplied