feat(apps): negotiated app-rendered form elicitations

Let a server hand a form `elicitation/create` to an MCP App and return the
app's ordinary `ElicitResult`, with the native elicitation form as the
fallback on every failure.

No second extension, no custom method, and no custom result shape: the only
new wire surface is a nested `elicitation` flag on the existing
`io.modelcontextprotocol/ui` extension on each side, plus
`_meta.ui.resourceUri` on the request. App rendering is selected only when
all four gates hold — client `elicitation.form`, client MCP Apps MIME type,
the nested `elicitation` setting on BOTH peers, and a valid absolute `ui://`
URI on the request.

Only the web client advertises the nested client-side setting, and only
because it has a sandbox renderer: supplying
`InspectorClientOptions.appElicitation` is what opts a client in, so CLI and
TUI keep advertising the MIME type without ever claiming they can resolve an
elicitation through an app.

Routing lives in the one funnel both entry points already use
(`enqueuePendingElicitation`), so the inbound handler and the MRTR driver
cannot diverge. Ownership is request-scoped — a per-request id keys the
renderer, iframe and bridge — so two concurrent elicitations can never
resolve through each other's bridges. An explicit `decline`/`cancel` is a
completed elicitation and goes back to the server; everything else (absent or
malformed metadata, resource/sandbox/bridge failure, an app with no
elicitation capability, a timeout, an invalid result or one that fails the
requested schema) falls back.

The Inspector speaks the ext-apps#733 / SEP-3118 wire protocol but cannot yet
consume its helpers — the released `@modelcontextprotocol/ext-apps` (1.7.5)
predates that PR. `core/mcp/appElicitation.ts` and
`AppRenderer/requestAppElicitation.ts` mirror it exactly and are marked for
deletion once a release containing it ships. One consequence needed its own
seam: ext-apps 1.7.5 parses the view's `ui/initialize` through a schema that
strips `elicitation`, so an app that correctly advertises it would look like
one that did not, silently turning every negotiated elicitation into a
fallback. `AppRenderer/appCapabilities.ts` records the raw frame instead, and
prefers the bridge's own value once it carries the key.

`AppRenderer` now takes an `AppRenderSource` union rather than a `Tool`, so
the same renderer and bridge factory serve an App tool and an elicitation
without either faking the other's shape.

Adds the public fixture (`app_choose_option` + a self-contained
`ui://demo/choose-option.html` app covering accept/decline/cancel), its two
showcase configs, and `smoke:web:elicit`, which drives the negotiated path
end to end in headless Chromium and then the same tool against a server that
never advertised the capability, asserting the native form takes it.

Closes #1854

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/elements/AppRenderer/appCapabilities.ts

@@ -0,0 +1,67 @@
+import type { AppBridge } from "@modelcontextprotocol/ext-apps/app-bridge";
+import type { Transport } from "@modelcontextprotocol/client";
+
+/**
+ * The app capabilities exactly as the view sent them, per bridge.
+ *
+ * `AppBridge.getAppCapabilities()` cannot be used for `elicitation` (#1854):
+ * ext-apps 1.7.5 parses the view's `ui/initialize` params through
+ * `McpUiAppCapabilitiesSchema`, a plain Zod object, so any key that schema does
+ * not declare is **stripped before the bridge stores it**. `elicitation` is
+ * exactly such a key — it is what ext-apps#733 adds — so an app that correctly
+ * advertises it looks, through the bridge's own accessor, like an app that did
+ * not. That silently turns every negotiated elicitation into a native-UI
+ * fallback, which is indistinguishable from "the feature is off".
+ *
+ * So the raw frame is observed on the way in and recorded here. A WeakMap keeps
+ * this per-bridge (never global) and lets the entry die with the bridge.
+ *
+ * Delete this module when ext-apps ships #733: `getAppCapabilities()` will
+ * carry `elicitation` itself, and {@link appAdvertisesElicitation} already
+ * prefers that value.
+ */
+const rawAppCapabilities = new WeakMap<AppBridge, Record<string, unknown>>();
+
+/** Shape of the `ui/initialize` frame this reads — nothing else is touched. */
+interface UiInitializeFrame {
+  method?: unknown;
+  params?: { appCapabilities?: unknown };
+}
+
+/**
+ * Wrap a connected bridge's transport so the view's `ui/initialize` params are
+ * recorded before the bridge parses them.
+ *
+ * Call AFTER `bridge.connect(transport)` — `connect` installs the handler this
+ * wraps. That ordering is safe: the view cannot send `ui/initialize` until the
+ * host has pushed its HTML into the sandbox, which happens later still.
+ */
+export function observeAppCapabilities(
+  bridge: AppBridge,
+  transport: Transport,
+): void {
+  const inner = transport.onmessage?.bind(transport);
+  transport.onmessage = (message, extra) => {
+    const frame = message as UiInitializeFrame;
+    if (frame.method === "ui/initialize") {
+      const advertised = frame.params?.appCapabilities;
+      if (typeof advertised === "object" && advertised !== null) {
+        rawAppCapabilities.set(bridge, advertised as Record<string, unknown>);
+      }
+    }
+    inner?.(message, extra);
+  };
+}
+
+/**
+ * Whether the app running on this bridge advertised the `elicitation`
+ * capability. Prefers the bridge's own accessor (correct once ext-apps#733
+ * ships) and falls back to the observed raw frame.
+ */
+export function appAdvertisesElicitation(bridge: AppBridge): boolean {
+  const parsed = bridge.getAppCapabilities() as
+    | Record<string, unknown>
+    | undefined;
+  if (parsed?.elicitation) return true;
+  return Boolean(rawAppCapabilities.get(bridge)?.elicitation);
+}