Add generic MCP App elicitation test fixture

[krubenok]

Goal

Add a reusable, server-agnostic Inspector test path for app-rendered form elicitations over the modern 2026-07-28 MRTR flow.

Fixture requirements

• Advertise request-scoped app elicitation client capabilities.
• Return input_required containing a complete native form request and _meta.ui.resourceUri.
• Serve a self-contained ui:// MCP App resource on the same connection.
• Forward the unchanged elicitation/create request into the App.
• Exercise accept, decline, and cancel standard elicitation responses.
• Retry the original tool call with unchanged arguments, echoed requestState, and inputResponses.
• Preserve legacy/native fail-closed behavior.

Documentation

Provide a concise manual testing recipe suitable for point-in-time SEP-3118 experiments and public examples, while keeping temporary compatibility details isolated in the experimental package.