[#6254]Improve: Rename tag to a name already exists in the current me…

…talake, it should return 409 instead of 500

### What changes were proposed in this pull request?
1. Rename tag to a name already exists in the current metalake, it should return 409 instead of 500
2. The error message should show the correct conflict name
### Why are the changes needed?
### Fix: #6254

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
ut

core/src/main/java/org/apache/gravitino/tag/TagManager.java

@@ -175,8 +175,8 @@ public Tag alterTag(String metalake, String name, TagChange... changes)
             throw new NoSuchTagException(
                 "Tag with name %s under metalake %s does not exist", name, metalake);
           } catch (EntityAlreadyExistsException e) {
-            throw new RuntimeException(
-                "Tag with name " + name + " under metalake " + metalake + " already exists");
+            throw new TagAlreadyExistsException(
+                "Tag with name %s under metalake %s already exists", getNewName(changes), metalake);
           } catch (IOException ioe) {
             LOG.error("Failed to alter tag {} under metalake {}", name, metalake, ioe);
             throw new RuntimeException(ioe);
@@ -352,6 +352,15 @@ public String[] associateTagsForMetadataObject(
                 }));
   }
 
+  private String getNewName(TagChange... changes) {
+    for (TagChange change : changes) {
+      if (change instanceof TagChange.RenameTag) {
+        return ((TagChange.RenameTag) change).getNewName();
+      }
+    }
+    return null;
+  }
+
   private TagEntity updateTagEntity(TagEntity tagEntity, TagChange... changes) {
     Map<String, String> props =
         tagEntity.properties() == null

core/src/test/java/org/apache/gravitino/tag/TestTagManager.java

@@ -343,6 +343,18 @@ public void testAlterTag() {
     Assertions.assertEquals("new comment", removedPropTag.comment());
     Map<String, String> expectedProp2 = ImmutableMap.of("k2", "v2");
     Assertions.assertEquals(expectedProp2, removedPropTag.properties());
+
+    // Test rename tag to an already-existing tag
+    tagManager.createTag(METALAKE, "tagA", "existing comment", null);
+    tagManager.createTag(METALAKE, "tagB", "some comment", null);
+
+    TagChange renameToExisting = TagChange.rename("tagB");
+    Exception e =
+        Assertions.assertThrows(
+            TagAlreadyExistsException.class,
+            () -> tagManager.alterTag(METALAKE, "tagA", renameToExisting));
+    Assertions.assertEquals(
+        "Tag with name tagB under metalake " + METALAKE + " already exists", e.getMessage());
   }
 
   @Test