Update ADK doc according to issue #1438 - 8

[adk-bot]

8. Document UI Widget support for MCP Tools

Doc file: docs/tools-custom/mcp-tools.md

Current state:

• Filtering (Optional): You can use the tool_filter parameter when creating an McpToolset to select a specific subset of tools from the MCP server, rather than exposing all of them to your agent.

Proposed Change:

• Filtering (Optional): You can use the tool_filter parameter when creating an McpToolset to select a specific subset of tools from the MCP server, rather than exposing all of them to your agent.

6. UI Rendering (Experimental): If an MCP tool defines a UI resource (via meta.ui.resourceUri), the ADK automatically detects it and signals the client to render the tool's UI widget. This allows for rich, interactive tool experiences.

Reasoning:
The McpTool now supports detecting mcp_app_resource_uri and sending a UiWidget event. This is a significant new feature for MCP Apps that should be documented.

Reference: src/google/adk/tools/mcp_tool/mcp_tool.py

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