feat(provider): add claude-fable-5 to vertex_ai_anthropic models

[akhil29]

The claude-fable-5 model was only added to the claude_code provider's static model list, but was omitted from the vertex_ai_anthropic provider. As a result, Claude Fable 5 does not appear in the model picker when using Forge via Vertex AI, even though the model is available on Vertex.

Mirror the claude_code definition into the vertex_ai_anthropic static models array so it surfaces for Vertex AI users.

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1 out of 2 committers have signed the CLA.

✅ amitksingh1490
❌ akhilapp

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