refactor(py/plugins/google-genai): move Veo to background model actions

[cabljac]

Fourth slice of the #5806 split: rewrites veo.py around create_veo_background_action on the shared background-model plumbing, wires Veo through BACKGROUND_MODEL/CHECK_OPERATION resolution in both GoogleAI and VertexAI (dropping the plain MODEL registration), and updates the google-genai-media sample to poll generate_operation directly. Touches only the Veo hunks of google.py; the Interactions wiring hunks follow in the next slice.

Part of the stack splitting #5806; recombined it reproduces that PR byte-for-byte. Co-authored with @huangjeff5.

Breaking changes

• Veo is now background-only on both GoogleAI and VertexAI: ai.generate(model='vertexai/veo-...') no longer resolves; use ai.generate_operation(...) + ai.check_operation(...) (see the updated google-genai-media sample).
• VeoModel and VeoConfigSchema are removed from genkit_google_genai.models.veo; use create_veo_background_action and VeoConfig.

[gemini-code-assist]

Code Review

This pull request refactors Veo video generation to use the new background model pattern and the GenAI generate_videos long-running API, updating both Google AI and Vertex AI plugins. It simplifies configuration schemas, updates sample code to use generate_operation and check_operation, and adapts the test suite. A critical bug was identified in from_veo_operation where operation.error is treated as a dictionary instead of a Pydantic model, which will cause an AttributeError at runtime.

[gemini-code-assist]

The operation.error object is an instance of the Pydantic model Status (from the google-genai SDK), which does not have a .get() method. Calling .get('message') on it will raise an AttributeError at runtime when an error is encountered. Instead, you should access the message attribute directly.

Suggested change

	if operation.error:
	op.error = Error(message=str(operation.error.get('message', 'Unknown error')))
	return op
	if operation.error:
	op.error = Error(message=operation.error.message or 'Unknown error')
	return op