fix(runtime): throw typed ExitError instead of generic Error for simulated exit (#97796) (#97803)

createNonExitingRuntime.exit() threw a generic Error, making it
impossible for upstream try-catch to distinguish a simulated process
exit from a real runtime crash. Added an ExitError class (internal-only,
not exported from SDK) that callers can check via instanceof.

ExitError is kept internal to avoid expanding the public plugin SDK
API surface without maintainer approval. Tests import directly from
./runtime.js.

Fixes #97796.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/runtime.test.ts

@@ -10,7 +10,7 @@ vi.mock("../packages/terminal-core/src/restore.js", () => ({
   restoreTerminalState: vi.fn(),
 }));
 
-import { createNonExitingRuntime, writeRuntimeJson } from "./runtime.js";
+import { createNonExitingRuntime, ExitError, writeRuntimeJson } from "./runtime.js";
 
 describe("createNonExitingRuntime", () => {
   it("returns runtime with exit function", () => {
@@ -18,14 +18,25 @@ describe("createNonExitingRuntime", () => {
     expect(typeof runtime.exit).toBe("function");
   });
 
-  it("exit function throws error", () => {
+  it("exit function throws ExitError", () => {
     const runtime = createNonExitingRuntime();
-    expect(() => runtime.exit(1)).toThrow("exit 1");
+    expect(() => runtime.exit(1)).toThrow(ExitError);
   });
 
-  it("exit function includes code in error message", () => {
+  it("ExitError includes exit code", () => {
     const runtime = createNonExitingRuntime();
     expect(() => runtime.exit(42)).toThrow("exit 42");
+    expect(() => runtime.exit(42)).toThrow(ExitError);
+  });
+
+  it("ExitError is distinguishable from generic Error", () => {
+    const runtime = createNonExitingRuntime();
+    try {
+      runtime.exit(1);
+    } catch (err) {
+      expect(err instanceof ExitError).toBe(true);
+      expect(err instanceof Error).toBe(true);
+    }
   });
 });
 

src/runtime.ts

@@ -95,11 +95,22 @@ export const defaultRuntime: OutputRuntimeEnv = {
   },
 };
 
+/** Error thrown by createNonExitingRuntime.exit() to signal simulated process exit. */
+export class ExitError extends Error {
+  constructor(
+    public readonly code: number,
+    message?: string,
+  ) {
+    super(message ?? `exit ${code}`);
+    this.name = "ExitError";
+  }
+}
+
 export function createNonExitingRuntime(): OutputRuntimeEnv {
   return {
     ...createRuntimeIo(),
     exit: (code: number) => {
-      throw new Error(`exit ${code}`);
+      throw new ExitError(code);
     },
   };
 }