fix(apps): restore MCP app sampling support reverted by #6933

[aharvard]

Problem

PR #6933 (local inference provider) was based on a stale branch that didn't include #7039's sampling changes to McpAppRenderer.tsx. When it merged ~16 hours after #7039, it silently reverted all client-side sampling plumbing.

What was lost

The following code from #7039 was removed from McpAppRenderer.tsx:

• RequestHandlerExtra import from @mcp-ui/client
• JSONRPCRequest import from @modelcontextprotocol/sdk/types.js
• SamplingCreateMessageParams / SamplingCreateMessageResponse type imports
• apiHost / secretKey state variables + initialization useEffect
• handleFallbackRequest callback (~35 lines) that handles sampling/createMessage requests
• onFallbackRequest={handleFallbackRequest} prop on <AppRenderer>

What survived

These parts of #7039 were not affected (different files):

• ✅ Server-side sampling route (crates/goose-server/src/routes/sampling.rs)
• ✅ Route wired up in mod.rs
• ✅ Sampling types in types.ts
• ✅ chat.html demo app + cache registration

Fix

This PR restores the exact code that #7039 added and #6933 removed. The @mcp-ui/client SDK (v6.1.0) still supports onFallbackRequest — confirmed in the bundled code.

Verification

• TypeScript compiles cleanly (tsc --noEmit passes)
• Diff is the exact inverse of feat: add local inference provider with llama.cpp backend and HuggingFace model management #6933's changes to McpAppRenderer.tsx

[Copilot]

Pull request overview

Restores the client-side MCP Apps “sampling” plumbing in the desktop UI host (McpAppRenderer) that was inadvertently reverted, so MCP Apps can again issue sampling/createMessage requests via the postMessage bridge and have them fulfilled by goose-server.

Changes:

• Re-adds the onFallbackRequest hook to handle sampling/createMessage JSON-RPC requests from the iframe.
• Re-introduces host-side initialization for apiHost and secretKey needed to call the sampling REST endpoint.
• Restores the required SDK/type imports for the fallback request handler.