feat: prevent variable resolution when prefixed with $ in server URL templates

[MelleD]

This PR updates the variable resolution logic in the OpenAPI Generator server template handling. Variables that are prefixed with a $ (e.g. ${server.servlet.context-path}) will no longer be resolved as OpenAPI template variables. Instead, they will be preserved as-is in the generated output.

fixes #22549

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Core-Team please have a look

@wing328
@jimschubert
@cbornet
@jmini
@etherealjoy

[wing328]

thanks for the pr

let's give it a try