Support empty media type objects (no schema, no examples)

Summary

OpenAPI specs with completely empty media type objects (application/json: {}) fail generation with "empty schema not implemented" error. This is a valid OpenAPI pattern for responses where the structure is unknown, unspecified, or intentionally flexible.

Specs Affected

• carriers/jitsu/specs/openapi.yaml - 19 locations with empty response content
• Common in real-world APIs for default error responses or flexible success responses

Current Behavior

Minimal Reproduction

openapi: 3.0.1
info:
  title: Empty Media Type Test
  version: 1.0.0
paths:
  /shipments:
    post:
      operationId: createShipment
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              properties:
                tracking_number:
                  type: string
      responses:
        default:
          description: Default response
          content:
            application/json: {}
        '200':
          description: Success
          content:
            application/json: {}

Error Output

Feature "empty schema" is not implemented yet.
	ignore_not_implemented: ["empty schema"]
to skip unsupported operations.

Impact

• Cannot generate complete clients for APIs with unspecified response structures
• Requires using ignore_not_implemented: ["empty schema"] workaround
• Skips entire operations rather than generating them with any type responses

Expected Behavior

When encountering application/json: {} (empty media type object):

1. Generate as any type:

   type CreateShipmentOK any
   type CreateShipmentDefault any

2. Or use empty struct for zero-schema responses:

   type CreateShipmentOK struct{}
   type CreateShipmentDefault struct{}

3. Allow configuration:

   generator:
     empty_schema_handling: "any"  # or "empty_struct" or "skip"

Use Cases

Empty media type objects are valid in OpenAPI 3.x for:

• Default error responses: Structure varies or is vendor-specific
• Flexible success responses: API returns different shapes based on context
• Legacy APIs: Documentation incomplete but API functional
• Passthrough endpoints: Proxy/gateway patterns

Real-World Example (Jitsu API)

paths:
  /v3/parcels/{id}:
    delete:
      operationId: deleteParcel
      responses:
        default:
          description: default response
          content:
            application/json: {}  # Error structure unspecified
        2XX:
          description: default response
          content:
            application/json: {}  # Success - may be empty or vary

Comparison with Upstream

Checked ogen-go/ogen - this pattern also fails there. This is a general OpenAPI spec handling gap.

Proposed Solution

Option 1: Generate as interface{}/any (Recommended)

• Treat {} media type as schema-less response
• Generate Go type as any (Go 1.18+) or interface{}
• Allows clients to decode arbitrary JSON

Option 2: Generate as empty struct

• For responses with no expected body
• Matches semantics of HTTP 204 No Content

Option 3: Make configurable

generator:
  empty_media_type_handling:
    mode: "any"  # or "empty_struct" or "skip"
    # Optional: per-status-code rules
    status_rules:
      default: "any"
      2XX: "empty_struct"

Workaround (Current)

Add to ogen.yml:

generator:
  ignore_not_implemented:
    - "empty schema"

Limitation: This skips the entire operation, preventing client generation for that endpoint.

OpenAPI Spec Compliance

Per OpenAPI 3.0.3 Media Type Object:

• schema field is OPTIONAL
• examples field is OPTIONAL
• Empty media type object {} is valid - means content is present but structure is unspecified

Test Command

ogen --target /tmp/test --clean empty-media-type-test.yaml

Expected: Generate client with any typed responses
Actual: Fails with "empty schema not implemented"