feat: Dynamic authentication handling in MCPToolset from ADK Readonly…

[pandasanjay]

…Context.state

This PR implements dynamic authentication handling in MCPToolset by passing authentication information (such as user-specific tokens) directly from the ADK session.state. With this approach, user tokens can be injected and sent from the ADK Context state to MCP, supporting user-specific authentication flows.

Key Improvements:

1. Dynamic Token Management:

   • MCPToolset now reads authentication information from the ADK ReadonlyContext.state at runtime, enabling dynamic, user-specific token management.
   • STDIO Transport: Authentication information is passed to MCP via environment variables. - #1402
   • Other Transports (SSE and HTTP Stream): MCPToolset now receives authentication details from the ADK ReadonlyContext.state, making them available for further transmission, enabling a unified authentication mechanism across all transports. - #805

2. Improved Unit Tests: Enhancements for better readability and maintainability.

3. New Sample: Demonstrates passing user tokens to MCP via environment variables, showcasing the dynamic flow from session.state.

4. Flexible & Secure Integration: Allows for more flexible and secure integration scenarios, supporting distinct authentication credentials per session or user.

[pandasanjay]

Hi @hangfei,

I've made the requested changes. Apologies for creating a new branch and pull request—I ran into some issues with the commit history on the previous one.

This pull request contains the latest code along with all the fixes.