fix(csharp): throw JsonException/NotSupportedException in SystemTextJson converters

[schani]

Bug

Per MS docs (https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/converters-how-to?pivots=dotnet-7-0#error-handling), custom System.Text.Json JsonConverters should throw JsonException (malformed/unexpected tokens) or NotSupportedException, so the serializer can attach JSON path/line/position diagnostics to the error. Generated C# SystemTextJson union converters instead threw a bare System.Exception, losing that diagnostic info for callers.

Root cause

Both throw sites in emitTransformation (the union-type read/write converter emitter) funneled through the emitThrow() helper in packages/quicktype-core/src/language/CSharp/SystemTextJsonCSharpRenderer.ts, which always emitted throw new Exception(...).

Fix

emitThrow() now takes an explicit exception type parameter:

• The Read (unmarshal) failure path now throws JsonException.
• The Write (marshal) failure path now throws NotSupportedException.

This only affects the SystemTextJson C# framework (--framework SystemTextJson); the NewtonSoft C# renderer is untouched.

Repro

node dist/index.js --lang csharp --src-lang json --framework SystemTextJson -o out.cs sample.json

with sample.json:

[
  {"kind": "alpha", "mixed": 5},
  {"kind": "beta", "mixed": "five"}
]

Before: throw new Exception("Cannot unmarshal type Mixed"); / throw new Exception("Cannot marshal type Mixed");
After: throw new JsonException("Cannot unmarshal type Mixed"); / throw new NotSupportedException("Cannot marshal type Mixed");

Test coverage

Since the existing end-to-end fixtures only check that the driver process exits nonzero on failure (any exception type satisfies that — they can't distinguish Exception from JsonException), this change is covered by a new unit test, test/unit/csharp-system-text-json-exceptions.test.ts, following the established "assert on generated source text" pattern (see test/unit/swift-final-classes.test.ts). It renders a schema producing a C# union with framework: SystemTextJson and asserts the generated source contains JsonException/NotSupportedException throws and no bare throw new Exception(.

Verification

• Confirmed the regression test fails against the pre-fix code.
• npm run build passes.
• npm run test:unit — all 164 tests pass.
• Manually re-ran the CLI repro above and confirmed the generated code now throws JsonException/NotSupportedException.
• The dotnet toolchain is not available in this environment, so the csharp-SystemTextJson end-to-end fixture (FIXTURE=csharp-SystemTextJson script/test) was not run locally; CI will validate that the generated code still compiles and the union round-trip fixtures still pass (they only assert nonzero exit on failure, so this change is expected to be compatible).

Fixes #2364

🤖 Generated with Claude Code