feat(tool): add return_direct feature for tool functions

[dylan-labs]

Title:
feat(tool): add return_direct feature for tool functions

Body:

Summary

• Add return_direct parameter that allows tool functions to return their output directly to the user without further LLM processing
• When return_direct=True, the agent stops the reasoning-acting loop immediately after tool execution

Changes

• Add return_direct field to ToolResponse class
• Add return_direct field to RegisteredToolFunction
• Add return_direct parameter to register_tool_function()
• Add return_direct parameter to register_mcp_client()
• Modify ReActAgent._acting() and reply() to handle return_direct logic
• Add example and documentation in examples/functionality/return_direct/

Usage

# Method 1: Set at registration time
toolkit.register_tool_function(
    get_weather,
    return_direct=True,
)

# Method 2: Set dynamically in ToolResponse
def my_tool(query: str) -> ToolResponse:
    return ToolResponse(
        content=[TextBlock(type="text", text="result")],
        return_direct=True,
    )

Closes #1077

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