Add UnavailableWebSearchTool to handle missing EXA_API_KEY gracefully

exa integrated

Swapped the browser’s Google search implementation from SerpApi to Serper and removed the SerpApi dependency path

addded file name headers

Add API key validation for GoogleSearchTool. Implemented a check to ensure the provided API keys are valid before initializing the search tool, enhancing error handling and user feedback for missing or invalid keys.

modularized app.py