docs(vertex): move model= from genai.Client() to from_genai()

[Goutham-Annem]

Summary

Fixes an incorrect code example in the Vertex AI migration guide where model= was passed to genai.Client() instead of from_genai().

The genai.Client() constructor does not accept a model parameter — passing it there is silently ignored. The model argument belongs to from_genai(), which uses it to set the default model for all requests.

Fixes #2416

Changes

• docs/integrations/vertex.md — corrected the "Option 2" example in the Migration to Google GenAI section

Before (incorrect):

client = from_genai(
    genai.Client(
        vertexai=True,
        project="your-project",
        location="us-central1",
        model="gemini-3-flash"   # ❌ not a valid Client() param
    )
)

After (correct):

client = from_genai(
    genai.Client(
        vertexai=True,
        project="your-project",
        location="us-central1",
    ),
    model="gemini-3-flash",   # ✅ belongs to from_genai()
)

Testing

Documentation-only change. Verified against the google-genai SDK source — genai.Client.__init__ does not accept a model parameter.

Note: This PR was developed with AI assistance.

[jxnl]

Consolidated and shipped in #2495. Closing this focused patch as superseded; thank you for the contribution.