[Phase 3b] MSSQL JSON - return json as string on read + OpenAPI/GraphQL tests

[souvikghosh04]

Why

Part of MSSQL native JSON support (#2768). Started as Phase 3b schema-discovery tests, but review (thanks @aaronburtle) surfaced a real product inconsistency that this PR now also fixes.

The problem: #2768 requires a JSON column to be treated as a normal string for input and output. But the MSSQL read path (FOR JSON PATH) was inlining a native json column as a nested JSON object. Consequences:

• REST returned "metadata": {"role":"admin"} (object) instead of "metadata": "{\"role\":\"admin\"}" (string).
• A GraphQL read of the String-typed field threw a GraphQLMapping error — the String leaf resolver calls JsonElement.GetString(), which fails on an object. So schema introspection could pass while profile_by_pk(id:1){ metadata } failed at runtime.
• OpenAPI/GraphQL advertised string, contradicting the object runtime — generated clients would mis-model the field.

What

Engine fix (MsSqlQueryBuilder): the read path now casts a native json column (SqlDbType.Json) to NVARCHAR(MAX) in WrappedColumns, so FOR JSON PATH emits it as an escaped JSON string rather than inlining it as an object. This is the single choke point for REST and GraphQL reads. Regular nvarchar columns are untouched. Mutations already return json as a string via the tabular OUTPUT clause, so no change was needed there.

Tests:

Layer	Test	Asserts
OpenAPI	JsonTypeSchemaTests	metadata is type: string with no format in the response and both request-body schemas (Profile_NoAutoPK for POST, Profile_NoPK for PUT/PATCH)
GraphQL introspection	MsSqlGraphQLJsonSchemaTests	Profile.metadata is the built-in String scalar (no custom JSON scalar)
GraphQL read (new)	MsSqlGraphQLJsonSchemaTests	profile_by_pk(id:1){ metadata } returns the payload as a JSON string — guards against introspection passing while a real read throws
REST	MsSqlRestJsonTypesTests (updated)	flipped back to the string contract: metadata is returned as a JSON string (ParseMetadata/GetJsonTypeList assert JsonValueKind.String)

Note: DAB's OpenAPI documentor does not emit Nullable on the property schema for any column type, so the schema tests assert type: string + no format (not nullability).

Intentionally omitted

• MCP describe_entities (T010): it projects only the config field name/description, not DB column data types, so there is no JSON-specific behavior to assert.

Notes

• This PR now includes a product (engine) change, not tests only.
• Requires SQL Server 2025 / Azure SQL (native json); CI already runs SQL 2025.
• Cannot be run locally (no SQL 2025 here) — relying on CI.

Delivery plan

Phase	What	Status
1	.NET 10 + SqlClient 6.x	✅ Merged (#3697/#3656)
2	JSON type + error mapping (engine)	✅ Merged (#3691)
3	Test fixture + REST CRUD tests	✅ Merged (#3720)
3b	json-as-string read fix + OpenAPI/GraphQL discovery & read tests (this PR)	🚧
4	Error/filter edge cases + regression + polish	Next

[Copilot]

Pull request overview

Adds schema-discovery test coverage for SQL Server 2025 native json columns to ensure DAB continues to expose them exactly like normal nullable string fields at the OpenAPI and GraphQL layers (no bespoke JSON scalar/format), aligning with the Phase 3b goals.

Changes:

• Adds an OpenAPI component-schema assertion that profiles.metadata is type: string, nullable: true, with no format.
• Adds a GraphQL introspection assertion that Profile.metadata is the built-in nullable String scalar (no custom JSON scalar).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Service.Tests/SqlTests/GraphQLQueryTests/MsSqlGraphQLJsonSchemaTests.cs	New GraphQL introspection test validating json columns map to built-in nullable String.
src/Service.Tests/OpenApiDocumentor/JsonTypeSchemaTests.cs	New OpenAPI generation test validating json columns are described as nullable string with no format.