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

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.

Reproduce the issue with the provided OpenAPI schema and Kotlin generator configuration, then run ./gradlew compileKotlin on the generated project. Trace how the components.schemas object named Timestamp becomes the generated class name and verify that the generated Kotlin declaration uses a valid custom model name rather than java.sql.Timestamp.