Tool call fix for Spring AI - Let ADK call tools

[alexcheng1982]

In the current implementation of Spring AI integration, tool calls are handled by Spring AI, this means the code needs to handle the conversion between Spring AI tools and ADK tools. This is unnecessary and introduces serval issues, like ToolContext is null (see #699 ), before and after tool callbacks are not invoked.

In this PR, tool calls are handled by ADK, just like other BaseLlm implementations.

• Spring AI tool calls execution is disabled by setting internalToolExecutionEnabled to false in ToolCallingChatOptions.
• FunctionToolCallback in Spring AI simply returns empty strings. FunctionToolCallback is only used to pass tool definitions. The actual tool calls are handled by ADK.
• Handles ToolResponseMessage in MessageConverter.
• Removed unused code and test.