feat: MCP app sampling support via stock SDK types (alt to #477) (#530)

* feat: MCP app sampling support via stock SDK types

Alternative to #477. Instead of forking a stripped-down sampling schema,
this reuses the stock `CreateMessageRequest` / `CreateMessageResult` /
`CreateMessageResultWithTools` types from `@modelcontextprotocol/sdk`
directly — same pattern already used for `tools/call`.

- Spec: `sampling/createMessage` listed under `### Standard MCP Messages`,
  `sampling?: { tools?: {} }` added to `HostCapabilities` (mirrors MCP
  `ClientCapabilities.sampling` shape for easy pass-through).
- SDK: `App.createSamplingMessage()` with overloads that narrow the return
  type based on whether `params.tools` is set; `AppBridge.oncreatesamplingmessage`
  setter; `CreateMessageRequest`/`CreateMessageResult*` added to the
  `AppRequest`/`AppResult` unions.
- Picks up SEP-1577 tool-calling support (`tools`, `toolChoice`, `tool_use`
  content blocks, `stopReason: "toolUse"`, array content) for free — unblocks
  the "nested agents" use case motivated by the original PR.

https://claude.ai/code/session_01ENGWTtsfcyP4S6fTWtUMAh

* feat(app): guard createSamplingMessage before handshake completes

---------

Co-authored-by: Claude <noreply@anthropic.com>

specification/draft/apps.mdx

@@ -516,6 +516,10 @@ UI iframes can use the following subset of standard MCP protocol messages:
 
 - `resources/read` - Read resource content
 
+**Sampling:**
+
+- `sampling/createMessage` - Request an LLM completion from the host (uses the standard MCP [`CreateMessageRequest`](https://modelcontextprotocol.io/specification/2025-11-25/client/sampling) / `CreateMessageResult` types, including SEP-1577 `tools` / `toolChoice` / `tool_use` content blocks). The host has full discretion over model selection and SHOULD apply rate limiting, cost controls, and user approval (human-in-the-loop). Apps MUST check `hostCapabilities.sampling` before sending this request, and `hostCapabilities.sampling.tools` before including `tools` in the request params.
+
 **Notifications:**
 
 - `notifications/message` - Log messages to host
@@ -662,6 +666,14 @@ interface HostCapabilities {
   };
   /** Host accepts log messages. */
   logging?: {};
+  /**
+   * Host supports LLM sampling (sampling/createMessage) from the view.
+   * Mirrors MCP ClientCapabilities.sampling so hosts can pass it through.
+   */
+  sampling?: {
+    /** Host supports tool use via `tools` and `toolChoice` params (SEP-1577). */
+    tools?: {};
+  };
   /** Sandbox configuration applied by the host. */
   sandbox?: {
     /** Permissions granted by the host (camera, microphone, geolocation, clipboard-write). */