fix: dart_mappable default enum value

[RootSoft]

This PR fixes enum generation for dart_mappable when unknown_enum_value: true is enabled.

Problem

Generated enums were emitting: @MappableEnum(defaultValue: 'unknown').

However, dart_mappable expects the actual enum case, not the serialized string value. Because of this, the generated mapper could not resolve the fallback enum index and still generated a throwing decode path for unknown backend values.

This caused older clients to crash when the backend introduced a new enum value.

Fix
The generator now emits the proper enum case reference instead of the encoded string value:

@MappableEnum(defaultValue: AnnouncementType.unknown)

This restores the expected dart_mappable behavior where unknown enum values safely fall back to the generated unknown enum case instead of throwing MapperException.unknownEnumValue(value).

Current behavior

It looks like @MappableEnum(defaultValue: ...) does not fallback when decoding an unknown enum value, and instead still throws MapperException.unknownEnumValue(...).

Example

This enum:

import 'package:dart_mappable/dart_mappable.dart';

part 'test.mapper.dart';

@MappableEnum(defaultValue: 'unknown')
enum Test {
  @MappableValue('info')
  info,

  @MappableValue('feature')
  feature,

  @MappableValue('unknown')
  unknown,
}

generates

@override
Test decode(dynamic value) {
  switch (value) {
    case 'info':
      return Test.info;
    case 'feature':
      return Test.feature;
    case 'unknown':
      return Test.unknown;
    default:
      throw MapperException.unknownEnumValue(value);
  }
}

Problem

If the backend later returns a new enum value, for example: { "type": "education" } decoding fails with: MapperException.unknownEnumValue(value) instead of falling back to: Test.unknown.

Expected behavior

When defaultValue is set, I would expect unknown enum values to decode to that default value, e.g.:

default:
  return Test.unknown;

This is especially important for API clients, where backend enums may evolve over time and older app versions should remain backward-compatible instead of crashing on new enum values.