fix: quote non-identifier dart_mappable enum values for nullable/list types

[RootSoft]

This PR fixes enum generation for dart_mappable when an enum's schema uses a JSON Schema type list such as ["string", "null"] (common in OpenAPI 3.1 specs, e.g. those generated from Laravel form request rules).

Problem

Generated dart_mappable enums were emitting the JSON value as a bare identifier instead of a string literal:

@MappableValue(hidden_gem)
hiddenGem,

@MappableValue(price_asc)
priceAsc,

@MappableValue(...) requires a constant expression, so an unquoted identifier like hidden_gem or price_asc is not valid Dart. The generated file fails to compile with:

Arguments of a constant creation must be constant expressions.

This only happened for enum values that are not valid Dart identifiers (contain underscores, dashes, digits, etc.) when the enum type was declared as a nullable/list type.

Root cause

The dart_mappable enum template only wrapped values in quotes when the enum's parsed type was exactly 'string':

@MappableValue(${type == 'string' ? "'$protectedJsonKey'" : protectedJsonKey})

When a schema declares:

"sort": {
  "type": ["string", "null"],
  "enum": ["rating", "name", "price_asc", "price_desc"]
}

the parser stores the enum type as the literal string "[string, null]" — not "string". (This is intentional: the nullability is preserved in the type string so toDartType() can derive String? for the JSON field.) Because "[string, null]" != "string", the condition was false and the value was emitted unquoted.

The freezed / json_serializable template already handled this correctly by falling back to quoting any non-numeric value. The dart_mappable template was missing that fallback.

Example

This schema:

"sort": {
  "type": ["string", "null"],
  "enum": ["rating", "name", "price_asc", "price_desc"]
}

previously generated (invalid):

@MappableEnum(defaultValue: Sort2.unknown)
enum Sort2 {
  @MappableValue(rating)
  rating,
  @MappableValue(name)
  name,
  @MappableValue(price_asc)
  priceAsc,
  @MappableValue(price_desc)
  priceDesc,
  @MappableValue(unknown)
  unknown;
  ...
}

Fix

The value-literal logic is now shared between the freezed and dart_mappable templates via a single helper. It quotes strings and any non-numeric value, leaves numeric values bare, and keeps null as null:

@MappableEnum(defaultValue: Sort2.unknown)
enum Sort2 {
  @MappableValue('rating')
  rating,
  @MappableValue('name')
  name,
  @MappableValue('price_asc')
  priceAsc,
  @MappableValue('price_desc')
  priceDesc,
  @MappableValue('unknown')
  unknown;
  ...
}

Behavior after the fix:

• String / non-identifier values → quoted: @MappableValue('price_asc'), @MappableValue('hidden_gem')
• Numeric enum values → bare: @MappableValue(0), @MappableValue(-1)
• Null values → @MappableValue(null)

This restores valid, compilable dart_mappable output for enums declared with nullable/list types.

Tests

Added a new e2e test enum_types_list_mappable (a dart_mappable variant of the existing enum_types_list fixture) that covers ["string", "null"], ["string", "integer"], and enum properties nested inside objects. The full e2e suite and generator/parser unit tests pass.