feat: Vertex AI support; add google-vertex and google-vertex-anthropic providers

[yukukotani]

Fix #272

Summary

• Add google-vertex provider with ADC-based autoload (project/location from env)
• Add google-vertex-anthropic provider to surface Anthropic Claude models via Vertex
• Patch dynamic import to load @ai-sdk/google-vertex/anthropic subpath under Bun

Details

• Autoload when GOOGLE_CLOUD_PROJECT (or GCP_PROJECT/GCLOUD_PROJECT) is present; uses GOOGLE_CLOUD_LOCATION or VERTEX_LOCATION (default us-central1)
• CLI auth guidance for Vertex (ADC / gcloud ADC)
• Dev dependency: @ai-sdk/google-vertex for typing; runtime still installs dynamically

Motivation

Unify Bedrock-like experience for Vertex: environment-based detection, seamless model listing (incl. Claude on Vertex) and minimal config.

Impact

• No breaking changes; providers are additive
• If Vertex env vars exist, providers will appear in list

Testing

• Typecheck passes
• Manual verification of model listing and selections under Vertex envs

[phase7]

Thanks for taking care of this one, eagerly waiting for this to get merged 🙏🏽

[rawwerks]

this would be really handy to have! maybe opencode can take care of it?

[rawwerks]

(and especially with gemini 3 dropping next week)

[rochacbruno]

thanks for this PR! I am waiting for this merge, so my team can try opencode.

[thdxr]

will merge this weekend

[yukukotani]

Ready