[BUG][Kotlin] Schema with object named Timestamp does not compile

[ross-paypay]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generator a client from a schema with a schema object named Timestamp invalid Kotlin code is generated.

data class java.sql.Timestamp (

    /* Milliseconds since the Unix epoch */
    @get:JsonProperty("epochMillis")
    val epochMillis: kotlin.String

)

The generator seems to be blindly using java.sql.Timestamp as the {{classname}} in template rendering even though it is defined as a custom schema.

openapi-generator version

7.12

OpenAPI declaration file content or url

info:
  title: OpenAPI definition
  version: v0
paths: {}
openapi: 3.0.1
servers:
  - url: http://localhost:8080
    description: Generated server url
components:
  schemas:
    Timestamp:
      type: object
      required:
        - epochMillis
      properties:
        epochMillis:
          type: string
          example: '1481899949606'
          description: Milliseconds since the Unix epoch
      description: A timestamp object representing a point on the ISO timeline in milliseconds since the Unix epoch.

Generation Details

generatorName: kotlin
library: jvm-okhttp4
additionalProperties:
  groupId: jp.ne.paypay
  useSpringBoot3: true
  serializationLibrary: jackson
  idea: true

Steps to reproduce

1. Generate the schema above using the openapi-generator config above
2. chmod +x gradlew
3. ./gradlew compileKotlin

Related issues/PRs

Suggest a fix

The issue appears to be that the CodegenModel's classname is being set to java.sql.Timestamp even though it should not.
I believe the fix should probably prevent that implicate behavior if the model is defined in the components.schemas.