Multipart generated enum case naming is inconsistent (other vs additionalProperties)

### Description

The generated code doesn't follow the multipart SOAR proposal, see details in https://github.com/apple/swift-openapi-generator/issues/596

### Reproduction

See the example in https://github.com/apple/swift-openapi-generator/issues/596.

### Package version(s)

On main.

### Expected behavior

Consistent behavior that matches the proposal.

### Environment

Xcode 16 Beta.

### Additional information

_No response_

Start by reading the multipart SOAR proposal and the reproduction linked in issue #596, then inspect where multipart enum cases are generated. Compare the `other` and `additionalProperties` cases with the proposal; the work is done when their naming is consistent with it.

Written by the indexing model from the issue text.