fix(codegen-csharp): expose field.string @dbColumnType:jsonb as a parsed JSON value (#98)

[dmealing]

What

A field.string carrying @dbColumnType: jsonb is the sanctioned open JSON bag: the physical column is jsonb (returns a parsed JSON value), but @dbColumnType is a physical override — the logical subtype stays string. The C# entity/DTO emitter typed the property off the logical subtype, so it emitted public string? Payload and Npgsql surfaced raw JSON text.

This changes the C# contract to expose a parsed JSON value at the API/wire boundary — uniform with TS z.unknown() (#97) and Python Any (#99). The minimal-API now returns/accepts a real JSON object, not a double-encoded string. This is an intended contract change (raw-text → parsed-value), and the gating runtime test was updated to match.

CLR type chosen: System.Text.Json.JsonDocument (+ rationale)

• Npgsql maps jsonb ↔ JsonDocument natively — and notably it is in Npgsql's built-in weakly-typed json mapping set, so it needs no EnableDynamicJson (the project never calls it; JsonNode/POCO dynamic mapping would have required it).
• Round-trips an arbitrary JSON object cleanly with EF Core 8 + Npgsql 8.0.10.
• System.Text.Json (de)serializes it as a real JSON value at the minimal-API boundary — a POST body deserializes into the property and a read serializes back out as the object, never an escaped string.

@dbColumnType:uuid / timestamp_with_tz are deliberately untouched — they keep their logical CLR type and convert at the DB seam (ADR-0013); only jsonb shifts the CLR type.

Changes

• CSharpNaming — DbColumnTypeClrOverride resolves the jsonb→JsonDocument override at the type-mapping seam; RequiresSystemTextJson drives a conditional using System.Text.Json;. Named constants only (DB_COLUMN_TYPE_JSONB, JsonbClrType) — no inline "jsonb".
• EntityGenerator — ScalarProperty applies the override; the entity / abstract-shape / value-object emit paths add the using when an own field is jsonb.
• DbContextGenerator — unchanged; still emits .HasColumnType("jsonb") (valid + explicit for a JsonDocument property).
• Committed integration fixture — Asset.g.cs regenerated via the drift harness (JsonDocument? Payload + the using).
• Runtime gate — RuntimeReturnTypeTests now asserts Asset.Payload is a parsed JsonDocument (object members a/b), not string.
• Persistence runner seam — EntityRow passes the JsonDocument through (not reflected as a POCO); Normalization routes its raw JSON through the existing sorted-key jsonb path, so the cross-port wire form stays byte-identical (payload: { a: 1, b: 2 }).

Tests (TDD)

New JsonbColumnCodegenTests (RED→GREEN): jsonb→JsonDocument?, @dbColumnType:uuid/plain field.string stay string?, jsonb still maps to a jsonb column, generated entity compiles.

Evidence (Docker available, Testcontainers round-trip DID run):

• Full C# suite green — Codegen 247 (+1 skipped regen harness), Conformance 666, Render 290, Cli 38.
• All 80 Docker/Testcontainers integration tests green, including RuntimeReturnTypeTests (asserts the parsed JsonDocument) and the persistence asset-uuid-roundtrip / normalization-wire-types scenarios (asset jsonb reads back as the sorted-key object end-to-end through EF Core + Npgsql against real Postgres).

Conformance

No registry change (jsonb legal-pairing already registered). api-contract corpus exercises no jsonb field — unaffected. Persistence corpus expected wire form is unchanged (still { a: 1, b: 2 }); only the C# runtime CLR type and runner seam changed to feed it.

Deferrals

C#-only (this issue's C# slice). Java/Kotlin/Python slices of #98 remain open.

Part of #98

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