task(Enable OS Test) Refs:#35131 (#35133)

## Summary

Registers `OpenSearchUpgradeSuite` as a dedicated CI test matrix entry
so it runs automatically with the OpenSearch 3.x upgrade container
activated. Also includes minor code-quality cleanup in
`ESContentletAPIImpl`. This is the CI companion to #35130,
which contains the actual test suite implementation.

## Changes

**CI — `.github/test-matrix.yml`**
- Added `Integration Tests - OpenSearch Upgrade Suite` suite entry under
`integration`
- Overrides `base_maven_args` with `-Dopensearch.upgrade.test=true`,
which activates the `opensearch-upgrade` Maven profile — that profile
starts the OpenSearch 3.x Docker container on port 9201 and restricts
Failsafe to run only `OpenSearchUpgradeSuite`
- Fixed a trailing whitespace in the existing `base_maven_args` block

**Backend — `ESContentletAPIImpl`**
- `categories.size() < 1` → `categories.isEmpty()`
- `StringBuffer` → `StringBuilder` (non-synchronized context)
- String concatenation inside loop → `StringBuilder.append()` chain

## Testing

The suite itself (`OpenSearchUpgradeSuite.java` and its member tests)
lives in the companion PR #35130 and must be merged before
this CI entry is exercisable end-to-end.

To verify the matrix entry locally:

```bash
# Requires the opensearch-upgrade profile in dotcms-integration/pom.xml
./mvnw verify -pl :dotcms-integration \
  -Dcoreit.test.skip=false \
  -Dopensearch.upgrade.test=true
```

The profile starts an OpenSearch 3.x container on port 9201 and limits
Failsafe to `OpenSearchUpgradeSuite`.

## Breaking Changes

**None.**

This PR fixes: #35131

Author: fabrizzio-dotCMS

.github/test-matrix.yml

@@ -59,7 +59,7 @@ test_types:
       needs_node: false
       # Base command for all integration tests
       base_maven_args: >-
-        verify -Dit.test.forkcount=1 -pl :dotcms-integration 
+        verify -Dit.test.forkcount=1 -pl :dotcms-integration
         -Dcoreit.test.skip=false
     suites:
       - name: "Integration Tests - MainSuite 1a"
@@ -80,6 +80,16 @@ test_types:
       - name: "Integration Tests - Junit5 Suite 1"
         test_class: "Junit5Suite1"
         stage_name: "IT Tests Junit5Suite1"
+      - name: "Integration Tests - OpenSearch Upgrade Suite"
+        test_class: "OpenSearchUpgradeSuite"
+        stage_name: "IT Tests OpenSearch Upgrade Suite"
+        # Override base_maven_args to activate the opensearch-upgrade Maven profile,
+        # which starts the opensearch-upgrade Docker container and restricts Failsafe
+        # to only run OpenSearchUpgradeSuite (see dotcms-integration/pom.xml:opensearch-upgrade)
+        base_maven_args: >-
+          verify -Dit.test.forkcount=1 -pl :dotcms-integration
+          -Dcoreit.test.skip=false
+          -Dopensearch.upgrade.test=true
 
   postman:
     condition_input: postman

dotCMS/src/main/java/com/dotcms/content/elasticsearch/business/ESContentletAPIImpl.java

@@ -7540,21 +7540,21 @@ public List<Contentlet> find(Category category, long languageId, boolean live, S
     public List<Contentlet> find(List<Category> categories, long languageId, boolean live,
             String orderBy, User user, boolean respectFrontendRoles)
             throws DotDataException, DotContentletStateException, DotSecurityException {
-        if (categories == null || categories.size() < 1) {
+        if (categories == null || categories.isEmpty()) {
             return new ArrayList<>();
         }
-        StringBuffer buffy = new StringBuffer();
+        StringBuilder buffy = new StringBuilder();
         buffy.append("+type:content +deleted:false");
         if (live) {
             buffy.append(" +live:true");
         } else {
             buffy.append(" +working:true");
         }
         if (languageId > 0) {
-            buffy.append(" +languageId:" + languageId);
+            buffy.append(" +languageId:").append(languageId);
         }
         for (Category category : categories) {
-            buffy.append(" +c" + category.getInode() + "c:on");
+            buffy.append(" +c").append(category.getInode()).append("c:on");
         }
         try {
             return search(buffy.toString(), 0, -1, orderBy, user, respectFrontendRoles);