Add app-rendered elicitation reference implementation

[krubenok]

Summary

• negotiate app-rendered form elicitations through core elicitation.form and the existing io.modelcontextprotocol/ui extension
• advertise the effective client capability set in modern 2026-07-28 request envelopes while retaining legacy initialize behavior
• bind each elicitation to its exact MCP App bridge and resolve it with the standard ElicitResult
• validate accepted content and fall back to the native form when app rendering or resolution fails
• exercise the MRTR retry with the same arguments, a new request id, inputResponses, and requestState
• include a generic self-contained test server and app with no product-specific details

The reference is pinned to ext-apps commit 89ab2bc, which contains the coordinated draft amendment.

Ecosystem references

• MCP Apps draft and TypeScript API: Add app-rendered elicitations to the MCP Apps draft modelcontextprotocol/ext-apps#733
• Inspector tracking issue: Implement negotiated app-rendered form elicitations modelcontextprotocol/inspector#1854
• C# SDK implementation: Add app-rendered elicitation support to MCP Apps modelcontextprotocol/csharp-sdk#1723
• Core SEP: SEP-3118: App-Rendered Elicitations for MCP Apps modelcontextprotocol/modelcontextprotocol#3118
• TypeScript SDK reference: Add app-rendered elicitation MRTR example modelcontextprotocol/typescript-sdk#2578

This PR intentionally targets krubenok/inspector:main. It is a public reference diff rather than an upstream Inspector contribution because the upstream repository currently routes external implementation work through issues.

Validation

• app-elicitation unit file: 10 passed
• modern-era integration file: 20 passed
• test-server TypeScript build passed
• core formatting check passed
• the earlier full reference-branch validation covered formatting, lint, typecheck, builds, and 4,824 web unit/integration tests