Expose public widget composition helpers

[faysou]

Summary

Downstream widget composition libraries need to recognize both classic AnyWidget instances and protocol-based widgets backed by MimeBundleDescriptor. The implementation already centralizes this behavior in _try_get_model_id, but it is private, so integrations duplicate incomplete checks.

This PR exposes two small public helpers:

• anywidget.get_model_id(value) returns the composition model ID for classic and protocol widgets.
• anywidget.is_widget(value) checks whether a value implements the composition protocol.

WidgetTrait and internal reference replacement now use the public resolver, while the private name remains as a compatibility alias. Tests cover protocol objects and plain values.

Validation

• uv sync --group dev
• uv run pytest (84 passed)
• git diff --check

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Latest commit: 3ee25f8

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