fix(agent): prevent session lock deadlock on timeout during compaction (#9855)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 64a28900f1
Co-authored-by: mverrilli <816450+mverrilli@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/agents/pi-embedded-runner/run/compaction-timeout.e2e.test.ts

@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import {
+  selectCompactionTimeoutSnapshot,
+  shouldFlagCompactionTimeout,
+} from "./compaction-timeout.js";
+
+describe("compaction-timeout helpers", () => {
+  it("flags compaction timeout consistently for internal and external timeout sources", () => {
+    const internalTimer = shouldFlagCompactionTimeout({
+      isTimeout: true,
+      isCompactionPendingOrRetrying: true,
+      isCompactionInFlight: false,
+    });
+    const externalAbort = shouldFlagCompactionTimeout({
+      isTimeout: true,
+      isCompactionPendingOrRetrying: true,
+      isCompactionInFlight: false,
+    });
+    expect(internalTimer).toBe(true);
+    expect(externalAbort).toBe(true);
+  });
+
+  it("does not flag when timeout is false", () => {
+    expect(
+      shouldFlagCompactionTimeout({
+        isTimeout: false,
+        isCompactionPendingOrRetrying: true,
+        isCompactionInFlight: true,
+      }),
+    ).toBe(false);
+  });
+
+  it("uses pre-compaction snapshot when compaction timeout occurs", () => {
+    const pre = [{ role: "assistant", content: "pre" }] as const;
+    const current = [{ role: "assistant", content: "current" }] as const;
+    const selected = selectCompactionTimeoutSnapshot({
+      timedOutDuringCompaction: true,
+      preCompactionSnapshot: [...pre],
+      preCompactionSessionId: "session-pre",
+      currentSnapshot: [...current],
+      currentSessionId: "session-current",
+    });
+    expect(selected.source).toBe("pre-compaction");
+    expect(selected.sessionIdUsed).toBe("session-pre");
+    expect(selected.messagesSnapshot).toEqual(pre);
+  });
+
+  it("falls back to current snapshot when pre-compaction snapshot is unavailable", () => {
+    const current = [{ role: "assistant", content: "current" }] as const;
+    const selected = selectCompactionTimeoutSnapshot({
+      timedOutDuringCompaction: true,
+      preCompactionSnapshot: null,
+      preCompactionSessionId: "session-pre",
+      currentSnapshot: [...current],
+      currentSessionId: "session-current",
+    });
+    expect(selected.source).toBe("current");
+    expect(selected.sessionIdUsed).toBe("session-current");
+    expect(selected.messagesSnapshot).toEqual(current);
+  });
+});