[dart-dio][json_serializable] fix unknownEnumValue for collection-typed enum properties

[marcelpinto]

enumUnknownDefaultCase is unusable with serializationLibrary=json_serializable whenever a spec contains a collection of enums. The template emits the fallback from the property's datatypeWithEnum, guarded on the property's isEnumOrRef. Both are wrong for collection-typed properties, in opposite directions.

1. List/Set of inline enums → invalid Dart. The property type is the container, so the template produces:

unknownEnumValue: List<EnumArraysArrayEnumEnum>.unknownDefaultOpenApi,

dart analyze on the current sample:

error - enum_arrays.dart:46:21 - The class 'List' doesn't have a constructor
        named 'unknownDefaultOpenApi'. - class_instantiation_access_to_member
error - enum_arrays.dart:46:21 - Arguments of a constant creation must be
        constant expressions. - const_with_non_constant_argument

Both errors are gone after this change, and no other diagnostic in the file changes. Three committed samples contain this today — enum_arrays.dart, object_with_inline_enum.dart, object_with_duplicate_inline_enum.dart — so it reproduces straight from bin/configs/dart-dio-petstore-client-lib-fake-json_serializable.yaml, which already sets enumUnknownDefaultCase: "true".

2. List of $ref'd enums → no fallback at all. isEnumOrRef is false on the property, so nothing is emitted and those fields still throw ArgumentError: `x` is not one of the supported values on an unknown value — the exact failure enumUnknownDefaultCase exists to prevent. No sample covers this shape, so it produces no sample diff; reproduce by pointing an array property at a $ref'd enum.

Fix. Key the guard and the type off items for collections, and off the property otherwise. json_serializable applies unknownValue: per element for iterables of enums, so the element type is the correct target in both cases — confirmed in the generated .g.dart:

availableMethods: (v as List<dynamic>)
    .map((e) => $enumDecode(_$RefundMethodEnumMap, e,
          unknownValue: RefundMethod.unknownDefaultOpenApi))
    .toList(),

uniqueItems: true properties (Set<…>) were affected identically and are fixed by the same branch.

Fixes #22160

PR checklist

• Read the contribution guidelines.
• Ran ./mvnw clean package, ./bin/generate-samples.sh, ./bin/utils/export_docs_generators.sh and committed all changed files. Sample diff is 3 lines across 3 files; export_docs_generators.sh produces no diff, as this is a template-only change that adds no option.
• @mention the technical committee: @yissachar @joernahrens @swipesight @jaumard

Note on verification: I confirmed the fix with dart analyze rather than a full build_runner run — the json_serializable sample fails build_runner with 46 FormatterExceptions on my machine both before and after this change, a pre-existing SDK / dart_style mismatch unrelated to it.

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Summary by cubic

Fixes enumUnknownDefaultCase in dart-dio with json_serializable for collection-typed enums. Generates valid Dart, applies per-element fallbacks for arrays/sets, and omits unsupported maps and nested collections.

• Bug Fixes
  • Use element type for unknownEnumValue on array/set properties; fall back to property type otherwise.
  • Drop invalid List<...>.unknownDefaultOpenApi / Set<...>.unknownDefaultOpenApi and omit unknownEnumValue on map and nested collection fields.
  • Emit fallbacks for $ref enums in arrays so unknown values no longer throw.
  • Template-only change in class.mustache; updated samples accordingly. Fixes [BUG] [DART-DIO] generator does not correctly handle unknownEnumValue if it is of type List #22160.

^{Written for commit df5ced5. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

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