Add Vertex AI provider with ADC support

- Implement google-vertex provider in packages/ai
- Support ADC (Application Default Credentials) via @google/generative-ai
- Add Gemini model catalog for Vertex AI
- Update packages/coding-agent to handle google-vertex provider

packages/ai/src/index.ts

@@ -2,6 +2,7 @@ export * from "./models.js";
 export * from "./providers/anthropic.js";
 export * from "./providers/google.js";
 export * from "./providers/google-gemini-cli.js";
+export * from "./providers/google-vertex.js";
 export * from "./providers/openai-completions.js";
 export * from "./providers/openai-responses.js";
 export * from "./stream.js";