Add support for JSON data type

[drmcclelland]

With the new JSON Data Type available in SQL Server 2025 and in Azure SQL, a column can contain JSON objects:

{"brand": "Dell", "specs": {"ram": "16GB", "storage": "512GB"}, "tags": ["electronics", "work"]}

and JSON arrays:

["Dell","Lenovo","Apple"]

Sample table data:

CREATE TABLE [dbo].[ProductCatalog](
	[ProductID] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY,
	[Attributes] [json] NOT NULL)

INSERT INTO [dbo].[ProductCatalog] (Attributes) VALUES ('{"brand": "Dell", "specs": {"ram": "16GB", "storage": "512GB"}, "tags": ["electronics", "work"]}')
INSERT INTO [dbo].[ProductCatalog] (Attributes) VALUES ('["Dell","Lenovo","Apple"]')

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This PR aims to resolve the following errors when executing the sp_generate_merge stored procedure:

EXEC [sp_generate_merge] 
     @Schema='dbo',
     @Table_Name='ProductCatalog'

1. Currently we get errors like this when a JSON column contains an object:

Msg 13639, Level 16, State 1, Line 1125
Target string size is too small to represent the JSON instance.

2. Even if we remove rows containing JSON objects from the table, we still get improperly generated SQL when a JSON column contains an array:

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With this fix, we get the following generated SQL:

SET IDENTITY_INSERT [dbo].[ProductCatalog] ON

MERGE INTO [dbo].[ProductCatalog] AS [Target]
USING (VALUES
  (2,CAST(N'["Dell","Lenovo","Apple"]' AS JSON))
 ,(3,CAST(N'{"brand":"Dell","specs":{"ram":"16GB","storage":"512GB"},"tags":["electronics","work"]}' AS JSON))
) AS [Source] ([ProductID],[Attributes])
ON ([Target].[ProductID] = [Source].[ProductID])
WHEN MATCHED AND EXISTS (SELECT CAST([Source].[Attributes] AS NVARCHAR(MAX))
                 EXCEPT  SELECT CAST([Target].[Attributes] AS NVARCHAR(MAX))) THEN
 UPDATE SET
  [Target].[Attributes] = [Source].[Attributes]
WHEN NOT MATCHED BY TARGET THEN
 INSERT([ProductID],[Attributes])
 VALUES([Source].[ProductID],[Source].[Attributes])
WHEN NOT MATCHED BY SOURCE THEN 
 DELETE;


SET IDENTITY_INSERT [dbo].[ProductCatalog] OFF