Add nonisolated to generated code for Swift 6.2 MainActor default isolation

[AnthonyMDev]

Summary

Closes apollographql/apollo-ios#3601

• Adds a new markTypesNonisolated option to ApolloCodegenConfiguration.OutputOptions that emits the nonisolated keyword on all generated type declarations
• Defaults to true when the codegen tool is compiled with Swift 6.2+ (#if compiler(>=6.2)), false otherwise
• Users on older Swift versions see no change; users on Swift 6.2+ get MainActor-compatible generated code automatically
• When enabled, generated types opt out of the module's default actor isolation, preventing compilation errors when the user's target sets SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor

Templates modified

OperationDefinitionTemplate, SelectionSetTemplate, FragmentTemplate, InputObjectTemplate, OneOfInputObjectTemplate, EnumTemplate, LocalCacheMutationDefinitionTemplate, SchemaMetadataTemplate, SchemaConfigurationTemplate, MockObjectTemplate, ObjectTemplate, InterfaceTemplate, UnionTemplate

Generated output (when enabled)

nonisolated public struct AllAnimalsQuery: GraphQLQuery {
  nonisolated public struct Data: AnimalKingdomAPI.SelectionSet {
    // ...
  }
}

Test plan

• Template unit tests verify nonisolated is emitted when option is true and omitted when false
• All regenerated test API code compiles (AnimalKingdomAPI, StarWarsAPI, GitHubAPI, SubscriptionAPI, UploadAPI)
• TestCodeGenConfigurations build and pass with nonisolated annotations
• Verify end-to-end with SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor on a test target

🤖 Generated with Claude Code

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