feat: Add vertex-anthropic provider to support Claude models on Google Vertex AI #1648
Description
The vertex provider currently only uses createVertex from @ai-sdk/google-vertex, which routes all requests through Google's publishers/google/models/...generateContent endpoint. This means specifying a Claude model (e.g., claude-sonnet-4-6) with provider: "vertex" fails because Claude models aren't served at that path. They require the publishers/anthropic endpoint.
The underlying SDK already ships with Anthropic support via @ai-sdk/google-vertex/anthropic (which exports createVertexAnthropic), but Task Master never imports or wires it up.
Expected behavior: Users with Google Cloud credentials should be able to use Claude models through Vertex AI, either via a new provider name (e.g., vertex-anthropic) or by auto-detecting Claude model IDs in the existing vertex provider and routing them through createVertexAnthropic.
Current behavior: Setting provider: "vertex" with modelId: "claude-sonnet-4-6" produces an API error because the request is sent to publishers/google/models/claude-sonnet-4-6@default:generateContent instead of the Anthropic endpoint.
Environment:
- Task Master v0.43.0
- @ai-sdk/google-vertex v3.0.113 (includes ./anthropic subpath export)