fix(extension): make tool cancellation cooperative (#89)

Tool cancellation is now cooperative end-to-end: the queue stays busy until compensation completes, cancellation is checked cooperatively across the pipeline, the fallback window is compensated too, and cleanup_failed is surfaced explicitly. Includes a gated delayed-compensation E2E test.

apps/extension/src/tools/session.ts

@@ -1,5 +1,6 @@
-import type { SessionManager } from "@/session-manager/manager";
+import { type SessionManager, SessionStartCleanupError } from "@/session-manager/manager";
 import type { RpcError } from "@/transport/types";
+import { rpcError } from "./errors";
 import { clearRecordingForSession } from "./record";
 import { isRpcError } from "./shared";
 import { returnBorrowedTab, type TabManagementDeps } from "./tabs";
@@ -58,6 +59,10 @@ export interface SessionStartResult {
   agent_window_id?: number;
 }
 
+export interface SessionStartDeps {
+  signal?: AbortSignal;
+}
+
 export interface SessionStopParams {
   session_id: string;
 }
@@ -91,6 +96,7 @@ export interface SessionStopDeps {
 export async function handleSessionStart(
   manager: SessionManager,
   params: SessionStartParams,
+  deps: SessionStartDeps = {},
 ): Promise<SessionStartResult | RpcError> {
   if (!params?.session_id) {
     return {
@@ -104,9 +110,23 @@ export async function handleSessionStart(
     const ctx = await manager.start(params.session_id, {
       size: sizeOrErr,
       focused: params.focused,
+      signal: deps.signal,
     });
     return { agent_window_id: ctx.agentWindowId };
   } catch (err) {
+    if (err instanceof SessionStartCleanupError) {
+      return rpcError("protocol_error", "cleanup_failed", err.message, {
+        resource_type: "agent_window",
+        resource_id: err.windowId,
+      });
+    }
+    if (
+      typeof err === "object" &&
+      err !== null &&
+      (err as { name?: string }).name === "AbortError"
+    ) {
+      return { code: "cancelled", message: "session_start aborted" };
+    }
     // chrome.windows.create / SessionManager failures are not CDP
     // failures (§4.5 reserves cdp_failed for raw CDP errors). Surface
     // them as protocol_error so the CLI maps to the right exit code