fix(child_process): remove self-referencing cycle in execSync/execFileSync errors

In `checkExecSyncError`, `ObjectAssign(err, ret)` copies `ret.error`
onto `err`, but `err` IS `ret.error`, creating `err.error === err`.
This causes `JSON.stringify(err)` to throw with a cyclic structure error.

Delete the `.error` property after the assign to break the cycle.

Closes #26844

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/node/child_process.ts

@@ -1016,6 +1016,9 @@ function checkExecSyncError(ret, args, cmd?) {
   if (ret.error) {
     err = ret.error;
     ObjectAssign(err, ret);
+    // ObjectAssign copies ret.error onto err, but err IS ret.error,
+    // creating a self-referencing cycle (err.error === err). Remove it.
+    delete err.error;
   } else if (ret.status !== 0) {
     let msg = "Command failed: ";
     msg += cmd || ArrayPrototypeJoin.$call(args, " ");

test/regression/issue/26844.test.ts

@@ -0,0 +1,24 @@
+import { expect, test } from "bun:test";
+import { execFileSync, execSync } from "child_process";
+
+test("execFileSync error should not have self-referencing cycle", () => {
+  try {
+    execFileSync("nonexistent_binary_xyz_123");
+    expect.unreachable();
+  } catch (err: any) {
+    // err.error should not be the same object as err (self-referencing cycle)
+    expect(err.error).not.toBe(err);
+    // JSON.stringify should not throw due to cyclic structure
+    expect(() => JSON.stringify(err)).not.toThrow();
+  }
+});
+
+test("execSync error should not have self-referencing cycle", () => {
+  try {
+    execSync("nonexistent_binary_xyz_123");
+    expect.unreachable();
+  } catch (err: any) {
+    expect(err.error).not.toBe(err);
+    expect(() => JSON.stringify(err)).not.toThrow();
+  }
+});