fix(proto-plus): annotate Message.serialize and Message.deserialize

[NorthIsUp]

MessageMeta.serialize() leaves instance unannotated, so pyright in strict mode reports Type of "serialize" is partially unknown at every call site. MessageMeta.deserialize() is annotated -> "Message", so MyMessage.deserialize(b) types as the base Message and callers have to isinstance-narrow it back to MyMessage before touching its fields.

• serialize(cls, instance: Union["Message", message.Message, Mapping[str, Any]]): the three shapes the docstring's "accepted by the type's constructor" covers.
• deserialize(cls: Type[_MessageT], payload: bytes) -> _MessageT: the same metaclass self-type pattern typeshed uses for EnumMeta. MyMessage.deserialize(b) now types as MyMessage.

Annotation-only, no runtime change. pytest tests in packages/proto-plus: 274 passed, 2 skipped. Checked with pyright strict against a generated google-ads message: GoogleAdsFailure.deserialize(b"") reveals GoogleAdsFailure, and serialize is fully known.

[google-cla]

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[gemini-code-assist]

Code Review

This pull request updates the type annotations in packages/proto-plus/proto/message.py. Specifically, it introduces a generic type variable _MessageT bound to Message and applies it to the deserialize method. It also adds type hints to the serialize method's instance parameter, allowing Message, message.Message, or a Mapping[str, Any]. There are no review comments, and I have no feedback to provide.