fix: avoid oneOf when @Accept adds default schema

[taba2424]

Describe the PR

• Fix OpenAPI v3 requestBody schema generation so the default schema created by @Accept does not get merged via oneOf with the actual body schema.
• Replace the placeholder type: object with the real body schema when @Param ... body is parsed after @Accept.
• Add a regression test covering @Accept json + body parameter.

Relation issue

N/A (no related issue)

Additional context

Example setup:

// @Accept json
// @Param request body types.CreateRequest true "作成リクエスト"

Command (example):

Generated specs using swag init with OpenAPI v3.1 output and YAML format.
Before, the generated requestBody schema included:

oneOf:
  - type: object
  - $ref: '#/components/schemas/CreateRequest'

This PR ensures the placeholder type: object from @accept is replaced by the actual body schema, resulting in only the $ref entry.