Fix flaky stderr race condition in ExternalParserTest nightly integration test

[Copilot]

ExternalParserTest.testInFileOutFile was intermittently failing with expected: <StdErrAfter> but was: <null> because SystemCommand reads stdout and stderr on separate threads, and execute() can return before the stderr listener thread has finished draining its final line.

Root cause

assertMetadata already had a retry loop for field2 (stdout_after) to tolerate this race, but field4 (stderr_after) had no such guard. Even after the stdout retry loop resolves field2, the stderr thread may still be processing StdErrAfter:field4.

Fix

Added an equivalent retry loop for field4 in assertMetadata:

// existing pattern for field2 (stdout_after)
var field2 = meta.getString("field2");
var maxAttempts = 10;
while (field2 == null && --maxAttempts >= 0) { Thread.sleep(50); ... }
Assertions.assertEquals("StdoutAfter", field2);

// new: same pattern for field4 (stderr_after)
var field4 = meta.getString("field4");
var maxAttemptsField4 = 10;
while (field4 == null && --maxAttemptsField4 >= 0) { Thread.sleep(50); ... }
Assertions.assertEquals("StdErrAfter", field4);