[BUG] dart-dio generator has basic deserialization issues

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

As far as I can tell, the description of the /users/latest endpoint 200 response is correct, it's just the User model... and the generator can't deserialize it because it's basically trying to cast a Map into an Iterable

DioError [DioErrorType.other]: Deserializing '{id: ba2be130-dece-4a5c-ac94-771fe70f9f7e, firstName: test, lastName: test, i...' to 'User' failed due to: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Iterable<Object?>' in type cast #0 BuiltJsonSerializers._deserialize (package:built_value/src/built_json_serializers.dart:178:11) #1 BuiltJsonSerializers.deserialize (package:built_value/src/built_json_serializers.dart:124:18)

openapi-generator version

6.2.0

OpenAPI declaration file content or url

{"openapi":"3.0.0","paths":{"/users/latest":{"get":{"operationId":"UserController_getLatestUser","summary":"","description":"","parameters":[],"responses":{"200":{"description":"","content":{"application/json":{"schema":{"$ref":"#/components/schemas/User"}}}}},"tags":["Users"],"security":[{"bearer":[]}]}},"/users":{"post":{"operationId":"UserController_createUser","summary":"","description":"","parameters":[],"responses":{"201":{"description":""}},"tags":["Users"],"security":[{"bearer":[]}]}}},"info":{"title":"api","description":"","version":"0.0.1","contact":{}},"tags":[],"servers":[],"components":{"schemas":{"User":{"type":"object","properties":{"id":{"type":"string"},"firstName":{"type":"string"},"lastName":{"type":"string"},"isActive":{"type":"boolean"}},"required":["id","firstName","lastName","isActive"]}}}}

Generation Details

Steps to reproduce

openapi-generator generate -g dart-dio -i http://localhost:3000/swagger-ui-json -o generated

Related issues/PRs

Suggest a fix

Reproduce the issue with the supplied OpenAPI declaration and the openapi-generator generate -g dart-dio command. Inspect the generated Dart client's handling of the User response and the deserialization path referenced in built_json_serializers.dart; done means the object response deserializes without the Map-to-Iterable error.