Fix OpenAPI import: additionalProperties: true should generate Record<unknown>

[Copilot]

Fixes an inconsistency in the OpenAPI import tool where additionalProperties: true and additionalProperties: {} generated different TypeSpec types, despite having identical semantic meaning in OpenAPI 3.1.

Problem

When importing an OpenAPI 3.1 specification with different additionalProperties values:

components:
  schemas:
    Foo:
      type: object
      properties:
        bar:
          type: object
          additionalProperties: true
        baz:
          type: object
          additionalProperties: {}

The conversion would produce inconsistent TypeSpec output:

model Foo {
  bar: {}                // ❌ Should be Record<unknown>
  baz: Record<unknown>   // ✅ Correct
}

Both additionalProperties: true and additionalProperties: {} semantically mean "additional properties are allowed with any type" in OpenAPI, so they should generate the same TypeSpec representation.

Solution

Updated the conversion logic in two places:

1. generate-types.ts: Modified the getObjectType method to explicitly handle additionalProperties: true by generating Record<unknown>

2. transform-component-schemas.ts: Updated the schema transformation to preserve additionalProperties: true during the conversion process

Result

Both scenarios now correctly generate Record<unknown>:

model Foo {
  bar: Record<unknown>   // ✅ Now correct for additionalProperties: true
  baz: Record<unknown>   // ✅ Still correct for additionalProperties: {}
}

Added comprehensive test coverage for both additionalProperties: true and additionalProperties: {} scenarios to prevent regression.

Fixes #[issue-number]

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Bug]: OpenAPI import tool - additional properties import issue</issue_title>
<issue_description>### Describe the bug

When importing an OpenAPI 3.1 description, additionalProperies: {} results in Record but additionalProperties: true results in {} type.

I'd expect both scenarios to result in the same exact TypeSpec definition since in both cases the OpenAPI semantics mean "additional properties are supported, we don't know their type thought"

Reproduction

components:
  schemas:
    Foo:
      type: object
      properties:
        bar:
          type: object
          additionalProperties: true
        baz:
          type: object
          additionalProperties: {}

Results in

model Foo {
  bar: {}
  baz: Record<unknown>
}

Where I'd expect instead

model Foo {
  bar: Record<unknown>
  baz: Record<unknown>
}

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #8583

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[github-actions]

All changed packages have been documented.

• ✅ @typespec/openapi3

Show changes

@typespec/openapi3 - fix ✏️

[importer] fixes import of additional properties : true {} to result in Record