feat(vertexai): integrate Claude model support

[CorieW]

Adds support for Claude models in VertexAI plugin

Testing

[gemini-code-assist]

Code Review

This pull request introduces support for Anthropic Claude models within the Genkit Vertex AI plugin. Key changes include the implementation of a VertexClaudeModelFactory to handle model interactions, updates to the plugin to list and resolve Anthropic publisher models, and new documentation and examples. Feedback focuses on critical technical refinements: ensuring image inputs are base64-encoded to meet API requirements, renaming internal tools to prevent user-defined conflicts, optimizing tool output encoding, and correcting non-standard regional endpoint URLs.

[cabljac]

can we have multiple system messages here?

[CorieW]

Could do, but consider:

genkit_openai: allows/preserves multiple system messages.
genkit_anthropic: only uses first, drops the rest.
genkit_google_genai: only uses first, drops the rest.
genkit_firebase_ai: only uses first, drops the rest.

Should we?

[CLAassistant]

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2 out of 4 committers have signed the CLA.

✅ CorieW
✅ kevmoo
❌ github-actions[bot]
❌ MichaelDoyle
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