Improved Elixir Atom Generation (#20229)

* Update ElixirClientCodegen.java

* Refine the regular expression for atoms

The original regex incorrectly matched `@atom` (unquoted atoms
cannot begin with @). However, through testing with `iex`, it also turns out that the atom
`:@` is legal.

The following atoms will now be quoted that would have been
incorrectly not quoted:

- `:@type`

* Add model to petstore client to test new atom creation regex pattern

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ElixirClientCodegen.java

@@ -48,7 +48,7 @@
 public class ElixirClientCodegen extends DefaultCodegen {
     private final Logger LOGGER = LoggerFactory.getLogger(ElixirClientCodegen.class);
 
-    private final Pattern simpleAtomPattern = Pattern.compile("\\A(?:(?:[_@\\p{Alpha}][_@\\p{Alnum}]*[?!]?)|-)\\z");
+    private final Pattern simpleAtomPattern = Pattern.compile("\\A(?:(?:[_\\p{Alpha}][_@\\p{Alnum}]*[?!]?)|-|@)\\z");
 
     @Setter protected String packageVersion = "1.0.0";
     @Setter protected String moduleName;