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_http_api/test_chunk_management_within_dataset/test_add_chunk.py

@@ -58,7 +58,7 @@ class TestAddChunk:
     @pytest.mark.parametrize(
         "payload, expected_code, expected_message",
         [
-            ({"content": None}, 100, """TypeError("unsupported operand type(s) for +: \'NoneType\' and \'str\'")"""),
+            ({"content": None}, 102, "`content` is required"),
             ({"content": ""}, 102, "`content` is required"),
             pytest.param(
                 {"content": 1},

test/testcases/test_http_api/test_chunk_management_within_dataset/test_update_chunk.py

@@ -48,12 +48,7 @@ class TestUpdatedChunk:
         "payload, expected_code, expected_message",
         [
             pytest.param({"content": None}, 0, "", marks=pytest.mark.skipif(os.getenv("DOC_ENGINE") == "infinity", reason="issues/6509")),
-            pytest.param(
-                {"content": ""},
-                100,
-                """APIRequestFailedError(\'Error code: 400, with error text {"error":{"code":"1213","message":"未正常接收到prompt参数。"}}\')""",
-                marks=pytest.mark.skip(reason="issues/6541"),
-            ),
+            ({"content": ""}, 102, "`content` is required"),
             pytest.param(
                 {"content": 1},
                 100,
@@ -61,12 +56,7 @@ class TestUpdatedChunk:
                 marks=pytest.mark.skip,
             ),
             ({"content": "update chunk"}, 0, ""),
-            pytest.param(
-                {"content": " "},
-                100,
-                """APIRequestFailedError(\'Error code: 400, with error text {"error":{"code":"1213","message":"未正常接收到prompt参数。"}}\')""",
-                marks=pytest.mark.skip(reason="issues/6541"),
-            ),
+            ({"content": " "}, 102, "`content` is required"),
             ({"content": "\n!?。；！？\"'"}, 0, ""),
         ],
     )