[BUG] [Rust Server] Not support free-form object code generated issue.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Not support free-form object code generated

openapi-generator version

OpenAPI declaration file content or url

modules/openapi-generator/src/test/resources/3_0/rust-server/openapi-v3.yaml
get:
tags: [Repo, Info]
operationId: Get Repo\Info
responses:
"200":
description: OK
content:
application/json:
schema:
$ref: "#/components/schemas/RepoObject"

RepoObject:
type: object

Command line used for generation

bin/rust-server-petstore.sh

Steps to reproduce

Run the generated code openapi-v3/

Related issues/PRs

No

Suggest a fix

Allow generate free-form object.

Start with modules/openapi-generator/src/test/resources/3_0/rust-server/openapi-v3.yaml and run bin/rust-server-petstore.sh to reproduce the generated Rust server output. Inspect how the free-form RepoObject schema is handled, then verify that generation succeeds for this schema and produces usable code in openapi-v3/.