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.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) {