fix(react): StrictMode cleanup + relax late-handler guard for re-reg (#631)

* fix(react): close App on useApp unmount; relax late-handler guard for re-registration

useApp's effect cleanup now closes the App (and its transport) instead
of only flipping a `mounted` flag. Under React StrictMode dev
double-invoke, the abandoned first instance's PostMessageTransport
otherwise keeps a window `message` listener and receives every host
postMessage alongside the second instance.

The late-handler guard from #629 now tracks `_everHadListener` per
event and only flags the *first* registration of a one-shot event
post-handshake. Re-registration (React useEffect cleanup + re-add on
dep change) of an event that had a pre-connect handler is silent,
including under `strict: true`.

* feat(app-bridge): warn on second ui/initialize

If a View double-mounts (e.g. React StrictMode in dev) without closing
the previous App, AppBridge now logs a console.warn when it receives a
second ui/initialize. Behavior is unchanged — it still responds and the
latest appInfo/appCapabilities replace the previous values — this is
purely a diagnostic for host implementers.

src/app-bridge.test.ts

@@ -15,6 +15,7 @@ import {
 import { z } from "zod/v4";
 
 import { App } from "./app";
+import { LATEST_PROTOCOL_VERSION } from "./types";
 import {
   AppBridge,
   getToolUiResourceUri,
@@ -2021,6 +2022,45 @@ describe("App <-> AppBridge integration", () => {
           expect(warnings()).toEqual([]);
         });
 
+        it("does not warn on re-registration when a handler existed pre-connect", async () => {
+          const h = () => {};
+          app.addEventListener("toolresult", h);
+          await bridge.connect(bridgeTransport);
+          await app.connect(appTransport);
+
+          // React-style: useEffect cleanup removes, dep change re-adds.
+          app.removeEventListener("toolresult", h);
+          app.addEventListener("toolresult", () => {});
+
+          expect(warnings().filter((m) => lateMsg.test(m))).toEqual([]);
+        });
+
+        it("warns once for the first late registration, not on subsequent ones", async () => {
+          await bridge.connect(bridgeTransport);
+          await app.connect(appTransport);
+
+          app.addEventListener("toolinput", () => {});
+          app.addEventListener("toolinput", () => {});
+          app.addEventListener("toolinput", () => {});
+
+          const late = warnings().filter((m) => lateMsg.test(m));
+          expect(late).toHaveLength(1);
+          expect(late[0]).toContain('"toolinput"');
+        });
+
+        it("tracks first-handler per event independently", async () => {
+          app.addEventListener("toolinput", () => {});
+          await bridge.connect(bridgeTransport);
+          await app.connect(appTransport);
+
+          app.addEventListener("toolinput", () => {}); // had pre-connect handler → silent
+          app.addEventListener("toolresult", () => {}); // first reg, late → warns
+
+          const late = warnings().filter((m) => lateMsg.test(m));
+          expect(late).toHaveLength(1);
+          expect(late[0]).toContain('"toolresult"');
+        });
+
         it("throws instead of warning when strict: true", async () => {
           const [strictAppT, strictBridgeT] =
             InMemoryTransport.createLinkedPair();
@@ -2034,6 +2074,9 @@ describe("App <-> AppBridge integration", () => {
             {},
             { autoResize: false, strict: true },
           );
+          // Pre-connect registration for toolcancelled — later swap must not throw.
+          strictApp.addEventListener("toolcancelled", () => {});
+
           await strictBridge.connect(strictBridgeT);
           await strictApp.connect(strictAppT);
 
@@ -2043,10 +2086,57 @@ describe("App <-> AppBridge integration", () => {
           expect(() => {
             strictApp.addEventListener("toolinput", () => {});
           }).toThrow(lateMsg);
-          expect(warnings()).toEqual([]);
+          // Swapping a handler that existed pre-connect is allowed under strict.
+          expect(() => {
+            strictApp.addEventListener("toolcancelled", () => {});
+          }).not.toThrow();
+          expect(warnings().filter((m) => lateMsg.test(m))).toEqual([]);
         });
       });
 
+      it("AppBridge warns on a second ui/initialize (View double-mount)", async () => {
+        await bridge.connect(bridgeTransport);
+        await app.connect(appTransport);
+        expect(warnings()).toEqual([]);
+
+        // Simulate a second View instance re-running the handshake.
+        appTransport.send({
+          jsonrpc: "2.0",
+          id: 99,
+          method: "ui/initialize",
+          params: {
+            protocolVersion: LATEST_PROTOCOL_VERSION,
+            appInfo: testAppInfo,
+            appCapabilities: {},
+          },
+        });
+        await flush();
+
+        const doubleInit = warnings().filter((m) =>
+          /second ui\/initialize/.test(m),
+        );
+        expect(doubleInit).toHaveLength(1);
+      });
+
+      it("close() stops further notification delivery (StrictMode cleanup relies on this)", async () => {
+        const received: unknown[] = [];
+        app.addEventListener("toolresult", (r) => received.push(r));
+        await bridge.connect(bridgeTransport);
+        await app.connect(appTransport);
+
+        await bridge.sendToolResult({
+          content: [{ type: "text", text: "before" }],
+        });
+        expect(received).toHaveLength(1);
+
+        await app.close();
+
+        await bridge
+          .sendToolResult({ content: [{ type: "text", text: "after" }] })
+          .catch(() => {});
+        expect(received).toHaveLength(1);
+      });
+
       it("AppBridge warns on tools/call from a View that skipped the handshake", async () => {
         bridge.oncalltool = async () => ({ content: [] });
         await bridge.connect(bridgeTransport);