Finish .js import extensions and drop the zod 3 fallback test

Add explicit .js extensions to the relative imports in app.ts,
app-bridge.ts and their tests so the emitted declarations resolve under
NodeNext (#704). Replace the zod 3 JSON Schema fallback test, whose
code path was removed, with the remaining non-Standard-JSON-Schema
rejection case as its own test.

src/app-bridge.test.ts

@@ -1551,40 +1551,9 @@ describe("App <-> AppBridge integration", () => {
         ).rejects.toThrow(/Invalid input for tool translate/);
       });
 
-      it("falls back to z.toJSONSchema for zod schemas lacking ~standard.jsonSchema (zod v3.25.x)", async () => {
-        // zod v3.25 implements ~standard.validate but not ~standard.jsonSchema.
-        // Simulate by stripping jsonSchema from a real zod schema.
-        const v4Schema = z.object({ q: z.string() });
-        const zod3LikeSchema = Object.assign(Object.create(v4Schema), {
-          "~standard": {
-            version: 1 as const,
-            vendor: "zod",
-            validate: v4Schema["~standard"].validate,
-            types: undefined as
-              | undefined
-              | {
-                  readonly input: { q: string };
-                  readonly output: { q: string };
-                },
-            // no jsonSchema
-          },
-        });
-
+      it("rejects listTools when a tool schema does not implement Standard JSON Schema", async () => {
         const appCapabilities = { tools: { listChanged: true } };
         app = new App(testAppInfo, appCapabilities, { autoResize: false });
-        app.registerTool(
-          "search",
-          { inputSchema: zod3LikeSchema },
-          async ({ q }: { q: string }) => ({
-            content: [{ type: "text" as const, text: q }],
-          }),
-        );
-        await app.connect(appTransport);
-
-        const list = await bridge.listTools({});
-        expect(list.tools[0].inputSchema.properties).toHaveProperty("q");
-
-        // Non-zod schema without jsonSchema → listTools rejects with guidance.
         app.registerTool(
           "broken",
           {
@@ -1598,6 +1567,8 @@ describe("App <-> AppBridge integration", () => {
           },
           async () => ({ content: [] }),
         );
+        await app.connect(appTransport);
+
         expect(bridge.listTools({})).rejects.toThrow(
           /does not implement Standard JSON Schema/,
         );

src/app.test.ts

@@ -6,13 +6,13 @@ import {
 } from "@modelcontextprotocol/server";
 import { z } from "zod/v4";
 
-import { App } from "./app";
+import { App } from "./app.js";
 import {
   LATEST_PROTOCOL_VERSION,
   McpUiInitializeRequestSchema,
   McpUiInitializeResultSchema,
   McpUiInitializedNotificationSchema,
-} from "./types";
+} from "./types.js";
 
 type ConnectedPair = {
   app: App;