[BUG] isString in enumVars is always `false`

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?

We get isString: false for ANY enum - be it a string one or int.

E.g. this one:

    GuardIntervalEnum:
      type: string
      enum:
        - '1_32'
        - '1_16'
        - '1_8'
        - '1_4'
        - '1_128'
        - '19_128'
        - '19_256'

makes this enumVars:

[{name=_132, isString=false, value='1_32'}, {name=_116, isString=false, value='1_16'}, {name=_18, isString=false, value='1_8'}, {name=_14, isString=false, value='1_4'}, {name=_1128, isString=false, value='1_128'}, {name=_19128, isString=false, value='19_128'}, {name=_19256, isString=false, value='19_256'}]`

Version: 5.3.1

Any ideas?

Start by reproducing the provided GuardIntervalEnum specification and tracing how OpenAPI Generator builds enumVars, especially the isString field. Done means string and non-string enum values produce the correct isString value, with coverage for the reported string enum case.