fix(apps): gate the capability observer on the bridge's own initialize schema

The hand-rolled accept check was shallower than the bridge's, so frames it
rejects could still set `elicitation` here — `appInfo: {}` passed, though
`McpUiInitializeRequestSchema` requires an implementation name and version.
Any approximation drifts; the schema itself does not, and ext-apps exports it.

Capabilities are still read from the ORIGINAL frame rather than the parse
output, since that schema is precisely what strips `elicitation` — validating
with it and reading through it would defeat the module. The one check the
schema cannot make is that the frame is a request at all (a notification
carries the same method and params), so the id check stays.

Also stops treating a truthy non-object as an advertisement: the draft declares
`elicitation` as an object, so `elicitation: true` is a value it does not
define and the bridge's schema will reject it the moment it carries the key.

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

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

@@ -57,6 +57,10 @@ describe("appCapabilities (#1854)", () => {
     expect(appAdvertisesElicitation(bridge)).toBe(true);
   });
 
+  it("is false when the bridge advertises nothing at all", () => {
+    expect(appAdvertisesElicitation(makeBridge(undefined))).toBe(false);
+  });
+
   it("is false for an app that advertised no elicitation", () => {
     const bridge = makeBridge({ availableDisplayModes: ["inline"] });
     const { transport } = makeTransport();
@@ -176,6 +180,40 @@ describe("appCapabilities (#1854)", () => {
     expect(appAdvertisesElicitation(bridge)).toBe(true);
   });
 
+  it("rejects a frame the bridge's own schema rejects", () => {
+    // `appInfo: {}` is missing the implementation name/version that
+    // `McpUiInitializeRequestSchema` requires, so the bridge answers an error
+    // and stores nothing — this gate must not be the laxer of the two.
+    const bridge = makeBridge({});
+    const { transport } = makeTransport();
+    observeAppCapabilities(bridge, transport);
+    transport.onmessage?.({
+      jsonrpc: "2.0",
+      id: 1,
+      method: "ui/initialize",
+      params: {
+        protocolVersion: "2026-01-26",
+        appInfo: {},
+        appCapabilities: { elicitation: {} },
+      },
+    } as unknown as JSONRPCMessage);
+    expect(appAdvertisesElicitation(bridge)).toBe(false);
+  });
+
+  it("does not treat a non-object elicitation value as an advertisement", () => {
+    // The draft declares `elicitation` as an object; `true` is a value it does
+    // not define, and truthiness would accept it.
+    const bridge = makeBridge({});
+    const { transport } = makeTransport();
+    observeAppCapabilities(bridge, transport);
+    transport.onmessage?.(initializeFrame({ elicitation: true }));
+    expect(appAdvertisesElicitation(bridge)).toBe(false);
+    transport.onmessage?.(initializeFrame({ elicitation: [] }));
+    expect(appAdvertisesElicitation(bridge)).toBe(false);
+    transport.onmessage?.(initializeFrame({ elicitation: {} }));
+    expect(appAdvertisesElicitation(bridge)).toBe(true);
+  });
+
   it("keeps bridges independent", () => {
     const a = makeBridge({});
     const b = makeBridge({});