fix(plugin): generate object examples from @example doc tags

[y-hsgw]

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?

With introspectComments enabled, an @example doc tag holding a JSON object is parsed successfully, but createLiteralFromAnyValue only handles arrays and primitives and returns undefined for objects. The resulting exception is swallowed by the property visitor, so the entire property (including required and type) is dropped from _OPENAPI_METADATA_FACTORY:

export class PresignedPostDto {
  /**
   * @example { "Content-Type": "video/mp4", "key": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" }
   */
  fields!: Record<string, string>;
}

// generated
static _OPENAPI_METADATA_FACTORY() {
    return {};
}

Issue Number: N/A

What is the new behavior?

createLiteralFromAnyValue now handles plain objects (and null) recursively, emitting object literal expressions with string literal keys so that non-identifier keys such as "Content-Type" work:

// generated
static _OPENAPI_METADATA_FACTORY() {
    return { fields: { required: true, type: "object", additionalProperties: { type: "string" }, example: { "Content-Type": "video/mp4", "key": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" } } };
}

Does this PR introduce a breaking change?

• Yes
• No

Other information

• Added coverage in test/plugin/fixtures/create-cat-alt2.dto.ts for an object @example on a Record<string, string> property.