Added JUnit Test Cases for Replacement

### Changes

This PR adds the missing unit test case for Replacement.

**Before:**
<img width="938" height="315" alt="Screenshot 2025-07-16 at 10 26 05 PM" src="https://github.com/user-attachments/assets/47a08af5-b240-40ba-8abf-9f20dce46c22" />

**After:**
<img width="947" height="315" alt="Screenshot 2025-07-16 at 10 24 41 PM" src="https://github.com/user-attachments/assets/2a18ffca-0b75-4e18-b071-7246c438ce29" />

### Test Plan

> mvn -Dtest=ReplacementTest test

```
...
[INFO] Running com.google.googlejavaformat.java.ReplacementTest
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.068 s - in com.google.googlejavaformat.java.ReplacementTest
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
```
----

> mvn test

```
[INFO] Results:
[INFO]
[INFO] Tests run: 1533, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  12.168 s
[INFO] Finished at: 2025-07-16T22:30:47-07:00
[INFO] ------------------------------------------------------------------------

```

### Next Steps

- Add more missing test cases to make this codebase more resilient.

Fixes #1268

COPYBARA_INTEGRATE_REVIEW=#1268 from surajdm123:add-test 1bad87c
PiperOrigin-RevId: 784313467

Author: surajdm123

core/src/test/java/com/google/googlejavaformat/java/ReplacementTest.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2025 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.google.googlejavaformat.java;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import com.google.common.collect.Range;
+import com.google.common.testing.EqualsTester;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** {@link Replacement}Test */
+@RunWith(JUnit4.class)
+public class ReplacementTest {
+
+  @Test
+  public void testCreateWithValidInput() {
+    Replacement replacement = Replacement.create(3, 7, "replacementText");
+    assertThat(replacement.getReplaceRange()).isEqualTo(Range.closedOpen(3, 7));
+    assertThat(replacement.getReplacementString()).isEqualTo("replacementText");
+  }
+
+  @Test
+  public void invalidReplacementRange() {
+    assertThrows(IllegalArgumentException.class, () -> Replacement.create(-1, 5, "text"));
+    assertThrows(IllegalArgumentException.class, () -> Replacement.create(10, 5, "text"));
+  }
+
+  @Test
+  public void testEqualsAndHashCode() {
+    new EqualsTester()
+        .addEqualityGroup(Replacement.create(0, 4, "abc"), Replacement.create(0, 4, "abc"))
+        .addEqualityGroup(Replacement.create(1, 4, "abc"))
+        .addEqualityGroup(Replacement.create(0, 4, "def"))
+        .testEquals();
+  }
+}