fix(go): include Type property in seen struct types

[hugoaguirre]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello @hugoaguirre, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue in the JSON schema inference logic where previously encountered struct types were not explicitly marked as "object" in the generated schema. By adding the type: "object" property, the generated schemas become more accurate and compliant, especially for recursive or shared struct definitions, improving the robustness of the schema generation process.

Highlights

• JSON Schema Generation for Seen Structs: Ensures that when a struct type is encountered multiple times during JSON schema inference (to prevent infinite recursion), the generated schema correctly includes "type": "object" along with "additionalProperties": true.

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[gemini-code-assist]

Code Review

This pull request aims to fix an issue where the JSON schema for a seen struct type was missing the type: "object" property. The change correctly adds this property in go/internal/base/json.go and updates/adds tests in go/internal/base/json_test.go to verify this behavior.

While the change is an improvement for standard structs, I've identified a significant edge case where it can produce an incorrect schema for structs that implement json.Marshaler. My review includes a comment with a suggested fix to handle this case correctly. Otherwise, the changes are logical and the tests are well-written.

[gemini-code-assist]

This change correctly adds Type: "object" for seen structs, making the schema more specific. However, this introduces an issue for struct types that implement json.Marshaler to produce a non-object JSON representation (e.g., a string or number).

For such types, this logic will incorrectly claim the type is object. For example:

type MyStringer struct { Value string }
func (s MyStringer) MarshalJSON() ([]byte, error) { return json.Marshal(s.Value) }

type Container struct {
    A MyStringer
    B MyStringer
}

When inferring the schema for Container, the schema for the second occurrence of MyStringer (field B) will be incorrectly marked as type: "object".

To fix this, we should check if the struct implements json.Marshaler and, if so, fall back to a more generic schema that doesn't specify the type, which is consistent with how jsonschema handles marshalers.

          var marshalerType = reflect.TypeOf((*json.Marshaler)(nil)).Elem()
          if baseType.Implements(marshalerType) || reflect.PtrTo(baseType).Implements(marshalerType) {
            return &jsonschema.Schema{
              AdditionalProperties: jsonschema.TrueSchema,
            }
          }
          return &jsonschema.Schema{
            Type:                 "object",
            AdditionalProperties: jsonschema.TrueSchema,
          }