KAFKA-18834 Fix LoggingResourceTest#testSetLevelDefaultScope (#19920)

- Flaky behavior
`LoggingResourceTest#testSetLevelDefaultScope` sometimes fails by not
capturing its expected WARN log.

- Root cause
Both `LoggersTest#testSetLevelWithValidRootLoggerNames` and
`LoggingResourceTest#testSetLevelDefaultScope` may share the same
`LoggerContext` when executed in the same JVM.
`LoggersTest#testSetLevelWithValidRootLoggerNames` calls
`loggers.setLevel("", ERROR)`, which mutates the global root logger
level to ERROR and suppresses WARN logs, which causes subsequent tests
to fail to emit WARN-level output.

- Fix in this PR
Resets the Log4j configuration after each test in `LoggersTest`,
ensuring that any global changes are reverted.

Reviewers: Chia-Ping Tsai <chia7712@gmail.com>

connect/runtime/src/test/java/org/apache/kafka/connect/runtime/LoggersTest.java

@@ -21,6 +21,8 @@ import org.apache.kafka.common.utils.Time;
 import org.apache.kafka.connect.runtime.rest.entities.LoggerLevel;
 
 import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.core.LoggerContext;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -47,6 +49,13 @@ public class LoggersTest {
         time = new MockTime(0, INITIAL_TIME, 0);
         loggers = (Loggers.Log4jLoggers) Loggers.newInstance(time);
     }
+    
+    @AfterEach
+    public void tearDown() {
+        // Reset LoggerContext to its initial configuration.
+        // This ensures any log level changes made in a test do not leak into subsequent tests.
+        LoggerContext.getContext(false).reconfigure();
+    }
 
     @Test
     public void testLevelWithNullLoggerName() {