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.ts

@@ -0,0 +1,54 @@
+import type { AgentMessage } from "@mariozechner/pi-agent-core";
+
+export type CompactionTimeoutSignal = {
+  isTimeout: boolean;
+  isCompactionPendingOrRetrying: boolean;
+  isCompactionInFlight: boolean;
+};
+
+export function shouldFlagCompactionTimeout(signal: CompactionTimeoutSignal): boolean {
+  if (!signal.isTimeout) {
+    return false;
+  }
+  return signal.isCompactionPendingOrRetrying || signal.isCompactionInFlight;
+}
+
+export type SnapshotSelectionParams = {
+  timedOutDuringCompaction: boolean;
+  preCompactionSnapshot: AgentMessage[] | null;
+  preCompactionSessionId: string;
+  currentSnapshot: AgentMessage[];
+  currentSessionId: string;
+};
+
+export type SnapshotSelection = {
+  messagesSnapshot: AgentMessage[];
+  sessionIdUsed: string;
+  source: "pre-compaction" | "current";
+};
+
+export function selectCompactionTimeoutSnapshot(
+  params: SnapshotSelectionParams,
+): SnapshotSelection {
+  if (!params.timedOutDuringCompaction) {
+    return {
+      messagesSnapshot: params.currentSnapshot,
+      sessionIdUsed: params.currentSessionId,
+      source: "current",
+    };
+  }
+
+  if (params.preCompactionSnapshot) {
+    return {
+      messagesSnapshot: params.preCompactionSnapshot,
+      sessionIdUsed: params.preCompactionSessionId,
+      source: "pre-compaction",
+    };
+  }
+
+  return {
+    messagesSnapshot: params.currentSnapshot,
+    sessionIdUsed: params.currentSessionId,
+    source: "current",
+  };
+}