[Vertex AI] Fix unsupported model name check introduced in #14610

[andrewheard]

The model name checks introduced in #14610 were checking that the model resource name, not the model name, started with gemini- or imagen-. This resulted in the log warning always being emitted since the model resource name (e.g., projects/my-project-id/locations/us-central-1/publishers/google/models/gemini-2.0-flash) never starts with gemini-.

This PR moves the check into VertexAI before the model name is converted to a model resource name. Also updated the parameter names in the GenerativeModel and ImagenModel constructors to be modelResourceName instead of name to reduce confusion.

#no-changelog

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[andrewheard]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses an issue where the model name check was incorrectly performed on the model resource name instead of the model name itself. The fix involves moving the check to the VertexAI class before the model name is converted to a resource name. Additionally, the parameter names in the GenerativeModel and ImagenModel constructors have been updated to modelResourceName to reduce confusion. Overall, this is a good fix that improves the correctness and clarity of the code.

Summary of Findings

• Model name validation: The model name validation was moved to the VertexAI class to ensure that the check is performed on the model name and not the model resource name.
• Parameter naming: The parameter names in the GenerativeModel and ImagenModel constructors were updated to modelResourceName to reduce confusion.

Merge Readiness

The pull request is ready to be merged. The changes address the issue described in the pull request description and improve the correctness and clarity of the code. I am unable to approve this pull request, and users should have others review and approve this code before merging.