fix(sandbox): improve docker image existence check error handling

Previously, `dockerImageExists` assumed any error from `docker image inspect` meant the image did not exist. This masked other errors like socket permission issues.

This change:
- Modifies `dockerImageExists` to inspect stderr when the exit code is non-zero.
- Returns `false` only if the error explicitly indicates "No such image" or "No such object".
- Throws an error with the stderr content for all other failures.
- Adds a reproduction test in `src/agents/sandbox/docker.test.ts`.

src/agents/sandbox/docker.test.ts

@@ -0,0 +1,44 @@
+import { spawn } from "node:child_process";
+import { describe, expect, it, vi, afterEach } from "vitest";
+import { EventEmitter } from "events";
+import { ensureDockerImage } from "./docker.js";
+
+vi.mock("node:child_process", () => ({
+  spawn: vi.fn(),
+}));
+
+describe("ensureDockerImage", () => {
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  function mockSpawn(exitCode: number, stdout: string, stderr: string) {
+    const child = new EventEmitter() as any;
+    child.stdout = new EventEmitter();
+    child.stderr = new EventEmitter();
+    (spawn as any).mockReturnValue(child);
+
+    setTimeout(() => {
+      child.stdout.emit("data", Buffer.from(stdout));
+      child.stderr.emit("data", Buffer.from(stderr));
+      child.emit("close", exitCode);
+    }, 10);
+    return child;
+  }
+
+  it("throws 'Sandbox image not found' when docker inspect fails with 'No such image'", async () => {
+    mockSpawn(1, "", "Error: No such image: test-image");
+
+    await expect(ensureDockerImage("test-image")).rejects.toThrow(
+      "Sandbox image not found: test-image. Build or pull it first."
+    );
+  });
+
+  it("throws 'Failed to inspect sandbox image' when docker inspect fails with other errors", async () => {
+    mockSpawn(1, "", "permission denied");
+
+    await expect(ensureDockerImage("test-image")).rejects.toThrow(
+      "Failed to inspect sandbox image: permission denied"
+    );
+  });
+});

src/agents/sandbox/docker.ts

@@ -50,7 +50,16 @@ async function dockerImageExists(image: string) {
   const result = await execDocker(["image", "inspect", image], {
     allowFailure: true,
   });
-  return result.code === 0;
+  if (result.code === 0) return true;
+  const stderr = result.stderr.trim();
+  if (
+    stderr.includes("No such image") ||
+    stderr.includes("No such object") ||
+    stderr.includes("not found")
+  ) {
+    return false;
+  }
+  throw new Error(`Failed to inspect sandbox image: ${stderr}`);
 }
 
 export async function ensureDockerImage(image: string) {