fix(anthropic): add anthropic-beta header for web-search on Vertex AI

[yamitzky]

• What kind of change does this PR introduce?
  • Bug fix
• What is the current behavior?
  • When using the Anthropic API via Vertex AI, attempting to use the web_search tool results in a 400 Bad Request error. The error message shows:

    tools.0: Input tag 'web_search_20250305' found using 'type' does not match any of the expected tags: 'bash_20250124', 'custom', 'text_editor_20250124',
    'text_editor_20250429', 'text_editor_20250728'
    

• What is the new behavior (if this is a feature change)?
  • By adding the anthropic-beta: web-search-2025-03-05 header to Vertex AI Anthropic API requests, the web_search tool now works correctly.
• Other information:
  • To use Anthropic beta features (such as web_search) via Vertex AI, the appropriate anthropic-beta header must be included. This change enables the web_search functionality on
    Vertex AI.
  • ref: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/partner-models/claude/web-search