feat: add Vertex AI Anthropic integration and setup documentation

[fg-nava]

Ticket

Resolve #156

Changes

• package.json: Added @ai-sdk/google-vertex and google-auth-library dependencies
• .env.example: Added Google Vertex AI configuration variables
• .gitignore: Added vertex-ai-credentials.json to ignore list
• docs/VERTEX_AI_ANTHROPIC_SETUP.md: Created comprehensive setup guide for Vertex AI Anthropic integration
• src/mastra/agents/web-automation-agent.ts: Added Vertex AI Anthropic provider import and configured model usage

Context for reviewers

This PR enables the use of Anthropic's Claude models through Google Vertex AI as an alternative to direct Anthropic API access. The setup process was somewhat involved, requiring specific GCP configurations, model enablement, and credential management, so detailed documentation was created to help future implementers.

The integration uses the recommended us-east5 region for best model availability and includes both CLI and console-based setup options for service account creation. The documentation references Anthropic's Claude Code setup guide which proved helpful despite being for a different use case.

Testing

In order to test this locally, you will need to follow the guide VERTEX_AI_ANTHROPIC_SETUP.md with access to our GCP project. Also need to pass VertexAI credentials.

[KayTV]

Nice job! 🎊