feat(mssql-json): enable JSON data type - type mapping and error codes (Phase 2)

Treat SQL Server 2025+ JSON columns exactly like string columns (2026-06-09 design, issue #2768). Engine-only change validated by unit tests.

- TypeHelper._sqlDbTypeToType[SqlDbType.Json] = typeof(string) (T004)

- SqlTypeConstants: register json as supported literal (T004)

- MsSqlDbExceptionParser BadRequestExceptionCodes += 13608-13614 so SQL JSON errors map to HTTP 400 (T006)

- unit tests for json type mapping and 400 error mapping (T005/T007)

The profiles DB fixture (T002) and Profile config entity (T003) are DEFERRED to Phase 3: the native json type requires SQL Server 2025+/Azure SQL and cannot be created on the LocalDB engine used by CI. They ship with the Phase 3 integration tests behind server-version gating.

No changes to PostgreSQL/MySQL/DwSql/CosmosDB (FR-012).

Author: souvikghosh04

src/Core/Models/SqlTypeConstants.cs

@@ -47,6 +47,7 @@ public static class SqlTypeConstants
         { "time", true },                 // SqlDbType.Time
         { "datetime2", true },            // SqlDbType.DateTime2
         { "datetimeoffset", true },       // SqlDbType.DateTimeOffset
+        { "json", true },                 // SqlDbType.Json (SQL Server 2025+) treated as string
         { "", false },                    // SqlDbType.Udt and SqlDbType.Structured provided by SQL as empty strings (unsupported)
         { "numeric", true}                // Not present in SqlDbType, however can be returned by sql functions like LAG and should map to decimal.
     };

src/Core/Resolvers/MsSqlDbExceptionParser.cs

@@ -30,7 +30,10 @@ public MsSqlDbExceptionParser(RuntimeConfigProvider configProvider) : base(confi
                 "4435", "4436", "4437", "4438", "4439", "4440", "4441",
                 "4442", "4443", "4444", "4445", "4446", "4447", "4448",
                 "4450", "4451", "4452", "4453", "4454", "4455", "4456",
-                "4457", "4933", "4934", "4936", "4988", "8102"
+                "4457", "4933", "4934", "4936", "4988", "8102",
+                // JSON data type validation errors (SQL Server 2025+). Invalid JSON
+                // supplied for a json column is a client error, mapped to HTTP 400.
+                "13608", "13609", "13610", "13611", "13612", "13613", "13614"
             });
 
             TransientExceptionCodes.UnionWith(new List<string>

src/Core/Services/TypeHelper.cs

@@ -97,6 +97,7 @@ public static class TypeHelper
             [SqlDbType.Float] = typeof(double),
             [SqlDbType.Image] = typeof(byte[]),
             [SqlDbType.Int] = typeof(int),
+            [SqlDbType.Json] = typeof(string),
             [SqlDbType.Money] = typeof(decimal),
             [SqlDbType.NChar] = typeof(char),
             [SqlDbType.NText] = typeof(string),

src/Service.Tests/OpenApiDocumentor/CLRtoJsonValueTypeUnitTests.cs

@@ -111,4 +111,17 @@ public void ResolveUnderlyingTypeForNullableValueType(Type nullableType)
         Assert.AreNotEqual(notExpected: JsonDataType.Undefined, actual: TypeHelper.GetJsonDataTypeFromSystemType(nullableType));
         Assert.IsNotNull(TypeHelper.GetDbTypeFromSystemType(nullableType));
     }
+
+    /// <summary>
+    /// Validates that the SQL Server 2025+ 'json' data type literal resolves to the
+    /// CLR string type and maps to JsonDataType.String, i.e. DAB treats a JSON column
+    /// just like a string column.
+    /// </summary>
+    [TestMethod]
+    public void JsonSqlDbTypeResolvesToString()
+    {
+        Type resolvedType = TypeHelper.GetSystemTypeFromSqlDbType("json");
+        Assert.AreEqual(typeof(string), resolvedType);
+        Assert.AreEqual(JsonDataType.String, TypeHelper.GetJsonDataTypeFromSystemType(resolvedType));
+    }
 }

src/Service.Tests/UnitTests/DbExceptionParserUnitTests.cs

@@ -94,5 +94,42 @@ public void TestIsTransientExceptionMethod(bool expected, int number)
 
             Assert.AreEqual(expected, dbExceptionParser.IsTransientException(SqlTestHelper.CreateSqlException(number)));
         }
+
+        /// <summary>
+        /// Validates that JSON data type validation error codes raised by SQL Server 2025+
+        /// when invalid JSON is supplied for a json column are mapped to HTTP 400 Bad Request.
+        /// </summary>
+        /// <param name="number">SQL error code populated in SqlException.Number.</param>
+        [DataTestMethod]
+        [DataRow(13608, DisplayName = "JSON validation error code 13608 maps to 400")]
+        [DataRow(13609, DisplayName = "JSON validation error code 13609 maps to 400")]
+        [DataRow(13610, DisplayName = "JSON validation error code 13610 maps to 400")]
+        [DataRow(13611, DisplayName = "JSON validation error code 13611 maps to 400")]
+        [DataRow(13612, DisplayName = "JSON validation error code 13612 maps to 400")]
+        [DataRow(13613, DisplayName = "JSON validation error code 13613 maps to 400")]
+        [DataRow(13614, DisplayName = "JSON validation error code 13614 maps to 400")]
+        public void TestJsonValidationErrorsMapToBadRequest(int number)
+        {
+            RuntimeConfig mockConfig = new(
+                Schema: "",
+                DataSource: new(DatabaseType.MSSQL, "", new()),
+                Runtime: new(
+                    Rest: new(),
+                    GraphQL: new(),
+                    Mcp: new(),
+                    Host: new(null, null, HostMode.Development)
+                ),
+                Entities: new(new Dictionary<string, Entity>())
+            );
+            MockFileSystem fileSystem = new();
+            fileSystem.AddFile(FileSystemRuntimeConfigLoader.DEFAULT_CONFIG_FILE_NAME, new MockFileData(mockConfig.ToJson()));
+            FileSystemRuntimeConfigLoader loader = new(fileSystem);
+            RuntimeConfigProvider provider = new(loader);
+            DbExceptionParser dbExceptionParser = new MsSqlDbExceptionParser(provider);
+
+            Assert.AreEqual(
+                System.Net.HttpStatusCode.BadRequest,
+                dbExceptionParser.GetHttpStatusCodeForException(SqlTestHelper.CreateSqlException(number)));
+        }
     }
 }