fix: replace brittle setTimeout(50) with polling waitForCondition in spawner tests

The agent-not-found fallback tests use fire-and-forget prompt chains
that must settle before assertions run. A fixed 50ms setTimeout is
insufficient on slower CI runners, causing intermittent failures.

Replace with waitForCondition() that polls at 10ms intervals with a
2s timeout — fast on green runs, reliable on slow CI.

Closes #3935

src/features/background-agent/spawner.test.ts

@@ -6,6 +6,25 @@ import {
   getSessionPromptParams,
 } from "../../shared/session-prompt-params-state"
 
+/**
+ * Poll until `fn()` returns true or timeout elapses.
+ * Replaces fixed `setTimeout(resolve, 50)` waits that cause flaky CI failures
+ * when the fire-and-forget prompt chain hasn't settled in time.
+ */
+async function waitForCondition(
+  fn: () => boolean,
+  timeoutMs = 2000,
+  intervalMs = 10,
+): Promise<void> {
+  const start = Date.now()
+  while (!fn()) {
+    if (Date.now() - start > timeoutMs) {
+      throw new Error(`waitForCondition timed out after ${timeoutMs}ms`)
+    }
+    await new Promise((r) => setTimeout(r, intervalMs))
+  }
+}
+
 describe("background-agent spawner agent-not-found fallback", () => {
   afterEach(() => {
     clearSessionPromptParams("session-fallback")
@@ -67,7 +86,7 @@ describe("background-agent spawner agent-not-found fallback", () => {
     await startTask(item as never, ctx as never)
 
     // Wait for the fire-and-forget prompt chain to settle
-    await new Promise(resolve => setTimeout(resolve, 50))
+    await waitForCondition(() => promptCalls.length >= 2)
 
     //#then
     // Should have called promptAsync twice: once with original agent, once with fallback
@@ -146,7 +165,7 @@ describe("background-agent spawner agent-not-found fallback", () => {
 
     //#when
     await startTask(item as never, ctx as never)
-    await new Promise(resolve => setTimeout(resolve, 50))
+    await waitForCondition(() => onTaskError.mock.calls.length > 0)
 
     //#then
     // Only one attempt — no retry for non-agent errors
@@ -199,7 +218,7 @@ describe("background-agent spawner agent-not-found fallback", () => {
 
     //#when
     await startTask(item as never, ctx as never)
-    await new Promise(resolve => setTimeout(resolve, 50))
+    await waitForCondition(() => onTaskError.mock.calls.length > 0)
 
     //#then
     // Verify retry was attempted (2 calls: original + fallback)
@@ -261,7 +280,7 @@ describe("background-agent spawner agent-not-found fallback", () => {
 
     //#when
     await startTask(item as never, ctx as never)
-    await new Promise(resolve => setTimeout(resolve, 50))
+    await waitForCondition(() => promptCalls.length >= 2)
 
     //#then
     expect(promptCalls).toHaveLength(2)
@@ -324,7 +343,7 @@ describe("background-agent spawner agent-not-found fallback", () => {
 
     //#when
     await startTask(item as never, ctx as never)
-    await new Promise(resolve => setTimeout(resolve, 50))
+    await waitForCondition(() => promptCalls.length >= 2)
 
     //#then
     expect(promptCalls).toHaveLength(2)