[BUG][Crystal] Uncompilable crystal code is generated

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

If the crystal generator is used, and the templates partial_model_generic.mustache, base_object.mustache or partial_oneof_module.mustache are instanced, we generate code using ....get_const(...) which isn't a thing in crystal, so the resulting code is unusable. (see https://github.com/crystal-lang/crystal/issues/8180 for proof of no const_get in crystal)

openapi-generator version

This seems to be from crystal introduction until latest master branch. (b3ec7faa8fc)

Reproducing

I encounter it when creating enum schemas, which is only used as a subtype in the petstore example which makes this not occur.

...
  content:
    schema:
      type: string
      enum:
      - "a"
      - "b"

Using some spec with the above contents trigger it.

Suggest a fix

The generator files mentioned should be fixed so it is not producing ruby code that doesn't work in crystal.

Start with partial_model_generic.mustache, base_object.mustache, and partial_oneof_module.mustache, then reproduce the issue using an enum schema with values "a" and "b". Inspect the generated Crystal code for get_const calls; done means the affected templates generate usable Crystal code without that unsupported call.