feat(swagger-plugin): add skipDefaultValues option to omit unspecified default fields and corresponding test

[mag123c]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

When using @ApiProperty({ enum: SubjectEnum, enumName: 'Subject' }) readonly caslSubjectType: SubjectEnum = this.constructor.name as SubjectEnum;, the generated OpenAPI schema always includes:

"__caslSubjectType__": {
  "default": "Function",
  "allOf": [
    { "$ref": "#/components/schemas/Subject" }
  ]
}

There is no built-in way to omit that default unless you explicitly set default: null.

Issue Number: #3338

What is the new behavior?

Introduce a skipDefaultValues option on the Swagger plugin.

Default: skipDefaultValues: false (existing behavior)

When enabled: any property without an explicit default initializer will not have a default field in the schema.

Example – with skipDefaultValues: true:

@ApiProperty({ enum: SubjectEnum, enumName: 'Subject' })
readonly __caslSubjectType__: SubjectEnum = this.constructor.name as SubjectEnum;

"__caslSubjectType__": {
  "allOf": [
    { "$ref": "#/components/schemas/Subject" }
  ]
}

Does this PR introduce a breaking change?

• Yes
• No

Other information

Docs will be added upon positive review and merge of this PR.

[kamilmysliwiec]

LGTM