Fix: reject empty/space-only content in update_chunk API (#14082)

Closes #6541

### What problem does this PR solve?

Add content validation to `update_chunk` (SDK and non-SDK) to reject
empty or whitespace-only content before it reaches the embedding model.

**Before:** Calling `update_chunk` with space-only content (like `" "`,
`""`, `"\n"`) bypassed validation and was sent directly to the embedding
model, which returned an error. This was the same bug previously fixed
for `add_chunk` in #6390, but `update_chunk` was missed.

**After:** Empty/whitespace-only content is caught by validation and
returns an error: `` `content` is required ``

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

test/testcases/test_web_api/test_chunk_app/test_chunk_routes_unit.py

@@ -220,6 +220,7 @@ def _load_chunk_module(monkeypatch):
 
     string_utils_mod = ModuleType("common.string_utils")
     string_utils_mod.remove_redundant_spaces = lambda text: " ".join(str(text).split())
+    string_utils_mod.is_content_empty = lambda content: content is None or not str(content).strip()
     monkeypatch.setitem(sys.modules, "common.string_utils", string_utils_mod)
 
     metadata_utils_mod = ModuleType("common.metadata_utils")

test/testcases/test_web_api/test_chunk_app/test_update_chunk.py

@@ -46,10 +46,10 @@ class TestUpdateChunk:
         "payload, expected_code, expected_message",
         [
             ({"content_with_weight": None}, 100, "TypeError('expected string or bytes-like object')"),
-            ({"content_with_weight": ""}, 100, """Exception('Error: 413 - {"error":"Input validation error: `inputs` cannot be empty","error_type":"Validation"}')"""),
+            ({"content_with_weight": ""}, 102, "`content_with_weight` is required"),
             ({"content_with_weight": 1}, 100, "TypeError('expected string or bytes-like object')"),
             ({"content_with_weight": "update chunk"}, 0, ""),
-            ({"content_with_weight": " "}, 0, ""),
+            ({"content_with_weight": " "}, 102, "`content_with_weight` is required"),
             ({"content_with_weight": "\n!?。；！？\"'"}, 0, ""),
         ],
     )