Add embedded MCP server for in-process UI introspection

[tilladam]

Summary

• Add an in-process MCP (Model Context Protocol) server to the Slint testing backend, allowing MCP clients (e.g. Claude Code, GitHub Copilot) to inspect and interact with a running Slint application's UI directly
• When the mcp feature is enabled and SLINT_MCP_PORT=<port> is set at runtime, an HTTP server starts on 127.0.0.1:<port> implementing MCP Streamable HTTP transport with 11 tools
• Refactors systest to share an IntrospectionState with the MCP transport, eliminating duplicated window/element tracking logic

Architecture

selector/mcp → testing/mcp
                 ├── introspection.rs  (shared state: arenas, properties, queries, conversions)
                 ├── mcp_server.rs     (HTTP server, JSON-RPC 2.0, 11 MCP tools)
                 └── systest.rs        (refactored to delegate to shared IntrospectionState)

MCP tools

Tool	Description
list_windows	Discover open windows
get_window_properties	Size, position, root element handle
get_element_tree	Flat list of UI hierarchy with properties
get_element_properties	Full details on a single element
find_elements_by_id	Look up elements by qualified ID
query_element_descendants	Pipeline-based search by type, ID, or role
take_screenshot	PNG screenshot as inline MCP image
click_element	Simulate mouse click
invoke_accessibility_action	Default, Increment, Decrement, Expand
set_element_value	Set text input content, slider value, etc.
dispatch_key_event	Send keyboard events

Key design decisions

• Shared introspection layer: element_properties(), query_element_descendants(), role/button/action conversions all live in introspection.rs, called by both transports.
• Localhost-only: Binds to 127.0.0.1, validates Origin header for DNS rebinding protection.
• Arena eviction: FIFO eviction with 10k handle cap; root handles preserved.
• Rich MCP instructions: The initialize response includes detailed workflow guidance, handle format, enum values, query syntax, and interaction tips — consumed by all MCP clients.

Test plan

• cargo build -p i-slint-backend-testing --features system-testing — systest compiles
• cargo build -p i-slint-backend-testing --features mcp — MCP compiles
• cargo test -p i-slint-backend-testing --features system-testing,mcp — 19 tests pass
• Manual integration test against gallery example: list windows, get tree, query 87 buttons, click, set text input value, take screenshots, dispatch key events — all working

[tronical]

Thanks, is is I think as reduced as it gets without "Handarbeit" :). I think we should take this in and start experimenting and learning :)

[tronical]

(Merging contingent on clippy being green, the servo failure is unrelated)