test(mssql-json): add malformed-JSON (400) and filter pass-through tests (Phase 4)

Covers the JSON error-handling and filter user stories from #2768:

- US7 REST: InsertMalformedJson_ReturnsBadRequest - posting invalid JSON for the json column is rejected by SQL Server and surfaced as HTTP 400 (not 500), exercising the 13608-13614 -> BadRequest mapping.

- US7 GraphQL: JsonColumn_GraphQLCreateWithMalformedJson_Fails - a createProfile mutation with malformed JSON fails with a GraphQL error (mirrors the merged vector-type pattern).

- US9 REST: FilterJsonColumnIsNotNull_Succeeds - filtering a json column as a string (metadata ne null) passes through to SQL and returns the 4 non-null rows.

Author: souvikghosh04

src/Service.Tests/SqlTests/GraphQLQueryTests/MsSqlGraphQLJsonSchemaTests.cs

@@ -82,5 +82,25 @@ public async Task JsonColumn_GraphQLRead_ReturnsPayloadAsString()
             Assert.AreEqual("admin", parsed.GetProperty("role").GetString());
             Assert.AreEqual(3, parsed.GetProperty("tier").GetInt32());
         }
+
+        /// <summary>
+        /// createProfile with malformed JSON in the metadata field must fail with a GraphQL error
+        /// (surfaced from SQL Server's json validation) rather than persisting invalid data.
+        /// </summary>
+        [TestMethod]
+        public async Task JsonColumn_GraphQLCreateWithMalformedJson_Fails()
+        {
+            string createMutationName = "createProfile";
+            string createMutation = @"mutation {
+                createProfile(item: { metadata: ""{ not valid json"" }) {
+                    id
+                    metadata
+                }
+            }";
+
+            JsonElement result = await ExecuteGraphQLRequestAsync(createMutation, createMutationName, isAuthenticated: false, expectsError: true);
+
+            SqlTestHelper.TestForErrorInGraphQLResponse(result.ToString());
+        }
     }
 }

src/Service.Tests/SqlTests/RestApiTests/MsSqlRestJsonTypesTests.cs

@@ -114,6 +114,21 @@ public async Task GetJsonTypeWithUnicode()
             Assert.AreEqual("éü😀", metadata.GetProperty("unicode").GetString());
         }
 
+        /// <summary>
+        /// GET /api/Profile?$filter=metadata ne null - Verify filtering a json column (treated as a
+        /// string) passes through to SQL: the 4 non-null rows match and the null row (id 5) does not.
+        /// </summary>
+        [TestMethod]
+        public async Task FilterJsonColumnIsNotNull_Succeeds()
+        {
+            HttpResponseMessage response = await HttpClient.GetAsync($"{JSON_TYPE_REST_PATH}?$filter=metadata ne null");
+            Assert.AreEqual(HttpStatusCode.OK, response.StatusCode, "Filtering a json column as a string should pass through and succeed.");
+
+            JsonElement items = JsonDocument.Parse(await response.Content.ReadAsStringAsync())
+                .RootElement.GetProperty("value");
+            Assert.AreEqual(4, items.GetArrayLength(), "Only the 4 rows with non-null metadata should match.");
+        }
+
         #endregion
 
         #region Write Tests
@@ -221,6 +236,26 @@ public async Task PatchJsonType_ToNull()
             }
         }
 
+        /// <summary>
+        /// POST /api/Profile - Verify that supplying invalid JSON for the json column is rejected by
+        /// SQL Server and surfaced as HTTP 400 (a client input error), not a 500. DAB treats the value
+        /// as a normal string, so JSON validation happens at the database boundary.
+        /// </summary>
+        [DataTestMethod]
+        [DataRow("{ \"metadata\": \"{ not valid json\" }", DisplayName = "Unclosed / unquoted object")]
+        [DataRow("{ \"metadata\": \"{\\\"key\\\": }\" }", DisplayName = "Missing value")]
+        public async Task InsertMalformedJson_ReturnsBadRequest(string requestBody)
+        {
+            HttpResponseMessage response = await HttpClient.PostAsync(
+                JSON_TYPE_REST_PATH,
+                new StringContent(requestBody, Encoding.UTF8, "application/json"));
+
+            Assert.AreEqual(
+                HttpStatusCode.BadRequest,
+                response.StatusCode,
+                "SQL Server rejects invalid JSON for a native json column; DAB must surface it as HTTP 400.");
+        }
+
         #endregion
 
         #region Helpers