[Winget-Software]

Closes #477.

This PR adds an opt-in freezed_nullable_presence option (default false). For Freezed models with JSON serialization enabled, it generates a <name>Present flag next to each optional nullable property. With the flag, toJson can tell an omitted key from an explicit null. A PATCH body needs that difference to clear a field without sending null for every other unset field. It is the Dart counterpart of OpenAPI Generator's openApiNullable / JsonNullable<T>. #477 has the background and links.

With the option enabled:

• Constructor: gets @Default(false) bool <name>Present for each property that is not required and is nullable (or references a nullable schema).
• Annotation: @Freezed(fromJson: false, toJson: false) together with a private const X._() constructor.
• fromJson/toJson: both are generated by hand. They delegate value conversion to a private @JsonSerializable() helper class (_XJson), so enum, date, nested-model and @JsonKey handling stay with json_serializable. fromJson sets each flag from json.containsKey(key). For a property that is null, toJson writes null when the flag is set and removes the key otherwise.
• JSON keys: use protectJsonKey, the same as @JsonKey(name: ...).

The option is ignored, and the current output is generated, when any of these is true:

• the option is off
• freezed_json_serialization: false
• the model is a union
• the model has no optional nullable properties

Changes:

• SWPConfig / GeneratorConfig / FillController / ProgrammingLanguage: read freezed_nullable_presence from YAML (inheriting from the root config) and pass it to the Freezed template, the same way freezed_json_serialization (using freezed required json_serializable #473) is wired.
• dart_freezed_dto_template.dart: presence-aware factory parameters, fromJson/toJson and helper class. The existing @JsonKey logic moved unchanged into _jsonKeyParams, so the helper class can reuse it.
• README.md: documents the option.
• CHANGELOG.md: adds a 1.46.0 entry. pubspec.yaml is left for the release commit, as in Fix properties becoming required when a sibling uses allOf #475.
• Tests:
  • swp_config_test.dart: default value and YAML parsing.
  • data_classes_test.dart: expected output for a model with an optional nullable allOf reference, a nullable string and a default field. It also checks that nothing changes when the option is off or freezed_json_serialization is false.
  • New patch_optional_nullable e2e test with a PATCH endpoint.

With the option off, generation is byte-identical to main, and every existing e2e expectation passes unchanged. The parser is not touched.

Known trade-offs, open to discussion:

• Flags on all optional properties: the parser treats every optional property as nullable, so properties that are optional but not nullable: true also get a flag. The flag is harmless, because it defaults to false and the field is omitted as before.
• copyWith: copyWith(notes: null) does not set notesPresent. To clear a field, set the flag as well: copyWith(notes: null, notesPresent: true).
• Name collision: a schema with both notes and a real notesPresent property would collide. I kept the name short and readable, but I can add a collision check or a configurable suffix if you prefer.

dart format, dart analyze and the full dart test suite (437 passed, 2 skipped) pass. I also enabled the option in a Flutter app with about 500 presence-tracked fields: build_runner generates everything without errors, and the app's test suite passes.

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