test: isolate base vitest thread blockers

Author: steipete

src/agents/pi-embedded-runner/compact.hooks.harness.ts

@@ -334,6 +334,47 @@ export async function loadCompactHooksHarness(): Promise<{
     splitSdkTools: vi.fn(() => ({ builtInTools: [], customTools: [] })),
   }));
 
+  vi.doMock("./compaction-safety-timeout.js", () => ({
+    compactWithSafetyTimeout: vi.fn(
+      async (
+        compact: () => Promise<unknown>,
+        _timeoutMs?: number,
+        opts?: { abortSignal?: AbortSignal; onCancel?: () => void },
+      ) => {
+        const abortSignal = opts?.abortSignal;
+        if (!abortSignal) {
+          return await compact();
+        }
+        const cancelAndCreateError = () => {
+          opts?.onCancel?.();
+          const reason = "reason" in abortSignal ? abortSignal.reason : undefined;
+          if (reason instanceof Error) {
+            return reason;
+          }
+          const err = new Error("aborted");
+          err.name = "AbortError";
+          return err;
+        };
+        if (abortSignal.aborted) {
+          throw cancelAndCreateError();
+        }
+        return await Promise.race([
+          compact(),
+          new Promise<never>((_, reject) => {
+            abortSignal.addEventListener(
+              "abort",
+              () => {
+                reject(cancelAndCreateError());
+              },
+              { once: true },
+            );
+          }),
+        ]);
+      },
+    ),
+    resolveCompactionTimeoutMs: vi.fn(() => 30_000),
+  }));
+
   vi.doMock("./wait-for-idle-before-flush.js", () => ({
     flushPendingToolResultsAfterIdle: vi.fn(async () => {}),
   }));