fix(google-vertex-anthropic): Fix incorrect NPM package used for Anthropic models used through Vertex

[MichaelYochpaz]

Summary

Fix the npm package path for the google-vertex-anthropic provider to use the correct subpath import.

Changes

• Changed npm from @ai-sdk/google-vertex to @ai-sdk/google-vertex/anthropic in providers/google-vertex-anthropic/provider.toml

Context

The google-vertex-anthropic provider requires the /anthropic subpath import from @ai-sdk/google-vertex for Claude models on Vertex AI to work correctly (Official docs reference).

Without this fix, thinking/reasoning capabilities don't work because:

1. The SDK key is incorrectly resolved to "google" instead of "anthropic"
2. Anthropic-specific message transforms (caching, thinking options) are not applied
3. The providerOptions get wrapped with the wrong key

This fixes anomalyco/opencode#9894

[MichaelYochpaz]

Hey @rekram1-node, sorry for the tag, but I (and anyone else using Vertex for Anthropic models) can't utilize Anthropic's models to their full capabilities with reasoning enabled without this issue fixed, and I use OpenCode daily for work.
Would appreciate if we could get it merged ASAP.

If there is anything you need me to change on the PR let me know.