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>

src/spec.types.ts

@@ -521,6 +521,14 @@ export interface McpUiHostCapabilities {
   updateModelContext?: McpUiSupportedContentBlockModalities;
   /** @description Host supports receiving content messages (ui/message) from the view. */
   message?: McpUiSupportedContentBlockModalities;
+  /**
+   * @description Host supports LLM sampling (sampling/createMessage) from the view.
+   * Mirrors the MCP `ClientCapabilities.sampling` shape so hosts can pass it through.
+   */
+  sampling?: {
+    /** @description Host supports tool use via `tools` and `toolChoice` parameters. */
+    tools?: {};
+  };
 }
 
 /**