fix(provider): enable thinking for google-vertex-anthropic models

Fixes reasoning/thinking not working for Claude models on GCP Vertex AI by correcting the npm package identifier and provider options key mapping.

The issue had two root causes:
1. models.dev API returns npm: '@ai-sdk/google-vertex' for google-vertex-anthropic provider, but variant generation expects '@ai-sdk/google-vertex/anthropic' (subpath import)
2. sdkKey() function didn't map '@ai-sdk/google-vertex/anthropic' to 'anthropic' key, causing thinking options to be wrapped with wrong provider key

Changes:
- Transform npm package to '@ai-sdk/google-vertex/anthropic' for google-vertex-anthropic provider in fromModelsDevModel()
- Add '@ai-sdk/google-vertex/anthropic' case to sdkKey() to return 'anthropic' key
- Add comprehensive tests for npm transformation, variant generation, and providerOptions key mapping