[Phase 4] MSSQL JSON - malformed-JSON (400) + filter edge-case tests

[souvikghosh04]

Resolves #3752

Why

Phase 4 of MSSQL native JSON support (#2768) — error-handling and filter edge cases. Builds on the merged engine + schema-discovery work (#3691, #3720, #3738).

What

User story	Test	Asserts
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 in MsSqlGraphQLVectorTypesTests)
US9 — REST	FilterJsonColumnIsNotNull_Succeeds	Filtering a json column as a string ($filter=metadata ne null) passes through to SQL and returns the 4 non-null rows

Because DAB treats json as a normal string and does no pre-validation, invalid JSON is caught at the database boundary and mapped to a client error — consistent with the "nothing special" contract.

Deferred (pending CI observation)

• Pruning 13608–13614: kept the full documented JSON-validation range for now. Removing a code that actually fires would regress it to a 500, so I'd rather confirm the exact code(s) SQL 2025 emits (via this PR's CI run) before narrowing the list.
• Sort / equality on a json column: SQL Server disallows comparing/sorting the native json type (except IS [NOT] NULL), so $orderby=metadata / metadata eq '…' behavior + error-code mapping needs CI confirmation before I assert on it.

Notes

• Tests only — no product code changes.
• 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 tests	✅ Merged (#3738)
4	Error / filter edge cases (this PR)	🚧

[Copilot]

Pull request overview

Adds Phase 4 integration coverage for SQL Server 2025 native json columns, validating malformed-JSON error handling and a REST filter edge case to ensure DAB continues treating json as a normal string while surfacing database validation errors as client-facing failures.

Changes:

• Added REST test coverage for $filter=metadata ne null (IS NOT NULL behavior) on json columns.
• Added REST test coverage ensuring malformed JSON inserts are surfaced as HTTP 400 (not 500).
• Added GraphQL mutation test coverage ensuring malformed JSON creates fail with a GraphQL error.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Service.Tests/SqlTests/RestApiTests/MsSqlRestJsonTypesTests.cs	Adds REST tests for json column filter IS NOT NULL behavior and HTTP 400 mapping on malformed JSON inserts.
src/Service.Tests/SqlTests/GraphQLQueryTests/MsSqlGraphQLJsonSchemaTests.cs	Adds GraphQL mutation test for malformed JSON create failure behavior.