Filter out empty API keys and improve tool calling support

This PR enhances API key validation by filtering out keys with empty values, preventing errors when attempting to use blank keys. It also significantly improves tool calling functionality across multiple providers:

• Added proper tool calling support for Anthropic, Bedrock, Cohere, and Vertex providers
• Implemented tool choice parameter handling for more control over tool selection
• Fixed handling of tool results in conversation history
• Added support for sequential tool calls and proper merging of tool results
• Improved image content handling in tool-related messages
• Added end-to-end test for tool calling functionality

The changes ensure consistent behavior across providers when using function calling capabilities, allowing for more complex AI assistant workflows that leverage external tools.