1. Partially reverts [go-server] Support min/max/defaults for values #15185. Reverts default value feature.

   Adding UnmarshalJSON breaks struct embedding. Any struct that has embedding will be empty after unmarshalling and API is unusable.

   Such can be reproduced with a model like such:

   title: A
   type: object
   allOf:
     - $ref: "./b.yaml"
   properties:
     value_a:
       type: string
   required:
     - value_a
   

   title: B
   type: object
   discriminator:
     propertyName: type
   properties:
     value_b:
       type: string
     type:
       type: string
   required:
     - value_b
   

   Generated:

   type A struct {
       B
   
       valueA string `json:"value_a,omitempty"`
   }
   
   // UnmarshalJSON sets *m to a copy of data while respecting defaults if specified.
   func (m *A) UnmarshalJSON(data []byte) error {
   
       type Alias A // To avoid infinite recursion
       return json.Unmarshal(data, (*Alias)(m))
   }

   type B struct {
       valueB string `json:"value_b,omitempty"`
   }
   
   // UnmarshalJSON sets *m to a copy of data while respecting defaults if specified.
   func (m *B) UnmarshalJSON(data []byte) error {
   
       type Alias B // To avoid infinite recursion
       return json.Unmarshal(data, (*Alias)(m))
   }

   valueA and valueB will be empty in Go.

2. Fix optional param errors with empty value in [go-server] Support min/max/defaults for values #15185

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