feat: update Google AI model options and refactor imports across agents

- Changed import from GoogleGenerativeAIProviderOptions to GoogleLanguageModelOptions in multiple files for consistency.
- Updated model references to 'google/gemini-3.1-flash-preview' and 'google/gemini-3.1-flash-lite-preview' in various agents.
- Removed unused imports and tools to streamline agent code.
- Added new tools for Google search and URL context in content strategist, copywriter, and research paper agents.
- Refactored agent configurations to improve clarity and maintainability.