fix: replace deprecated pkg_resources with pkgutil.extend_path

[cortexuvula]

Problem

The lark_oapi/ws/pb/google/__init__.py file triggers a deprecation warning on every import:

UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30.

Solution

Replace pkg_resources.declare_namespace() with pkgutil.extend_path() directly.

The original code already used pkgutil.extend_path as a fallback when pkg_resources wasn't installed, so this change is functionally equivalent — just removes the deprecated path entirely.

Impact

• No behavior change — pkgutil.extend_path serves the same namespace package purpose
• Eliminates the deprecation warning for all users on setuptools ≥81
• Future-proofs against pkg_resources removal

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