[Phase 2] Enable MSSQL JSON data type - type mapping & error codes (engine)

[souvikghosh04]

Phase 2 — MSSQL JSON engine enablement

Part of the phased delivery for MSSQL JSON data type support (issue #2768).

Phase	Scope	Status
Phase 1	.NET 10 + Microsoft.Data.SqlClient 6.x upgrade (#3656)	✅ Merged
Phase 2	JSON engine enablement (this PR)	🚧 Draft
Phase 3	DB fixture + CRUD integration tests (REST / GraphQL / MCP)	Pending
Phase 4	Error handling, filter/orderby pass-through, edge cases & regression	Pending

What this delivers

Treats a SQL Server 2025+ JSON column exactly like a string column (per the 2026-06-09 design). This PR is the engine-only change, validated entirely by unit tests (no database dependency).

Production change (2 lines):

• TypeHelper._sqlDbTypeToType[SqlDbType.Json] = typeof(string) — JSON columns flow through every existing string read/write path. (T004)
• MsSqlDbExceptionParser.BadRequestExceptionCodes += 13608–13614 — SQL Server JSON-validation errors surface as HTTP 400 instead of 500. (T006)

Supporting changes:

• SqlTypeConstants: register json as a supported SqlDbType literal. (T004)
• Unit tests: json → string type mapping, and JSON error codes → 400. (T005 / T007)

The 13608–13614 error-code range is the starting set; it is pruned to the numbers SQL Server actually raises in Phase 4 (via the malformed-write / rejected-filter integration tests).

Why the DB fixture is NOT in this PR

The profiles test table (T002) and the Profile config entity (T003) are deferred to Phase 3. The native json column type requires SQL Server 2025+ / Azure SQL, and the CI test job initializes DatabaseSchema-MsSql.sql against LocalDB (SQL 2022 max), which does not know the json type — CREATE TABLE profiles (metadata json ...) fails schema init for the entire MsSql suite. Since no Phase 2 test consumes that table, the fixture ships with the Phase 3 integration tests behind server-version gating (see T031). This keeps Phase 2 green on the existing CI engine.

Requirements covered

FR-004, FR-007, FR-014 (schema-surface FR-001 is exercised by the Phase 3 integration tests).

Non-goals / out of scope

• No changes to PostgreSQL, MySQL, DwSql, or CosmosDB (FR-012).
• No new GraphQL scalar, no OData operator allow-list, no MCP annotation (superseded 2026-06-09).
• DB fixture + integration tests (REST/GraphQL/MCP) land in Phase 3; error/filter/edge coverage in Phase 4.

Testing

• dotnet build src/Service.Tests/Azure.DataApiBuilder.Service.Tests.csproj — succeeds, 0 warnings.
• New unit tests pass: JsonSqlDbTypeResolvesToString, TestJsonValidationErrorsMapToBadRequest (8 cases).
• No database-backed tests in this PR; full TestCategory=MsSql integration run arrives with Phase 3 (requires SQL Server 2025+).

Draft rationale

Opened as draft — the feature is not functionally verifiable end-to-end until the Phase 3 integration tests run against a SQL Server 2025+ environment, and the error-code list is finalized in Phase 4.