Feat: update and add new tests for web api apps (#12714)

### What problem does this PR solve?

This PR adds missing web API tests (system, search, KB, LLM, plugin,
connector). It also addresses a contract mismatch that was causing test
failures: metadata updates did not persist new keys (update‑only
behavior).

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Other (please describe): Test coverage expansion and test helper
instrumentation

api/db/services/document_service.py

@@ -824,11 +824,18 @@ class DocumentService(CommonService):
             changed = False
             for upd in updates:
                 key = upd.get("key")
-                if not key or key not in meta:
+                if not key:
                     continue
 
                 new_value = upd.get("value")
                 match_provided = "match" in upd
+                if key not in meta:
+                    if match_provided:
+                        continue
+                    meta[key] = dedupe_list(new_value) if isinstance(new_value, list) else new_value
+                    changed = True
+                    continue
+
                 if isinstance(meta[key], list):
                     if not match_provided:
                         if isinstance(new_value, list):