fix: define model literal and add types prefix in URL Context example

[xiaolai]

Automated audit: This PR was generated by NLPM, a natural language programming linter. Please evaluate the diff on its merits.

The Url Context code sample in skills/vertex-ai-api-dev/references/structured_and_tools.md (the block under ## Url Context) has two issues that prevent it from running as written:

1. model=model_id — model_id is never defined in the snippet (every other code block in the same file uses a literal model string). Copy-pasting this example raises NameError: name 'model_id' is not defined.
2. config=GenerateContentConfig(...) — the file imports the SDK as from google.genai import types, so the constructor must be qualified as types.GenerateContentConfig(...). The four sibling examples in the same file (Structured Output, Function Calling, Search Grounding, Code Execution) all use the qualified form.

The fix matches the rest of the file:

response = client.models.generate_content(
    model="gemini-3-flash-preview",
    contents="Compare recipes from http://example.com and http://example2.com",
    config=types.GenerateContentConfig(
        tools=[types.Tool(url_context=types.UrlContext)],
    )
)

The other code blocks in the same file already use gemini-3-flash-preview, so it's the conservative pick for consistency.

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[gemini-code-assist]

Code Review

This pull request updates a Python code example in the documentation for Vertex AI. Specifically, it replaces a generic model variable with a specific model ID ('gemini-3-flash-preview') and corrects the reference to 'GenerateContentConfig' by adding the 'types' prefix. I have no feedback to provide.

[xiaolai]

@googlebot I signed it!

[markmcd]

No longer needed.