fix(csharp): omit superfluous usings when no converter class is emitted

[schani]

Bug

In --lang cs, the NewtonSoft and System.Text.Json renderers unconditionally
emitted the usings needed for the date-time converter class whenever
attributes (--features attributes-only, or the default full mode) were
requested — even for models with no date/date-time fields, named unions, or
enums, i.e. cases where no converter class is generated at all.

Repro (--lang cs --features attributes-only --framework NewtonSoft on a
model with only string/number/bool fields):

using System.Globalization;
using Newtonsoft.Json;
using Newtonsoft.Json.Converters;

System.Globalization and Newtonsoft.Json.Converters are only used inside
the generated Converter/date-converter class, which wasn't being emitted
here — so both usings were dead code.

Root cause

NewtonsoftCSharpRenderer.emitUsings() and
SystemTextJsonCSharpRenderer.emitUsings() emitted System.Globalization
(and, for NewtonSoft, Newtonsoft.Json.Converters) unconditionally whenever
attributes or helpers were needed, instead of checking whether the converter
class itself (emitConverterClass()) was actually going to be emitted.

Fix

Added a shared needConverterClass getter in each renderer (the same
condition already used to decide whether to call emitConverterClass()) and
gated the System.Globalization / Newtonsoft.Json.Converters usings on it.
Newtonsoft.Json (needed for JsonProperty attributes) and the
System.Text.Json/System.Text.Json.Serialization usings are unaffected and
still always emitted when needed.

Test coverage

• test/unit/csharp-superfluous-usings.test.ts (new): asserts that
  attributes-only output for a model with no dates/unions/enums omits
  System.Globalization and Newtonsoft.Json.Converters for both the
  NewtonSoft and System.Text.Json frameworks, while still keeping the
  required namespace usings; also asserts the usings are still present when a
  converter class is actually generated (full/helpers mode).
• test/languages.ts: enabled attributes-only fixture coverage for
  simple-object.json under both CSharpLanguage (NewtonSoft) and
  CSharpLanguageSystemTextJson, so the end-to-end fixture pipeline also
  exercises this code path.

Verification

• npm run build passes.
• npx vitest run test/unit — 174/174 tests pass, including the 4 new ones.
• Manually re-ran the original repro (node dist/index.js --lang cs --features attributes-only --framework NewtonSoft sample.json) and
  confirmed the superfluous usings are gone, while a model with a date field
  or the full (non-attributes-only) mode still emits the converter usings
  correctly.
• The C# fixture tests (FIXTURE=csharp / FIXTURE=csharp-systemtextjson)
  could not be run locally because dotnet is not installed in this
  environment; the new simple-object.json / attributes-only fixture
  combination added in test/languages.ts will be exercised by CI.

Fixes #800.

🤖 Generated with Claude Code