[rust] Fixed discriminator causing duplicate enum variants

[ranger-ross]

This is one of the issues for #20500.

The issue is that when a discriminator has multiple "types" that refer to the same schema.
ie.

discriminator:
   propertyName: type
   mappings:
      student: '#/components/schemas/Person'
      teacher: '#/components/schemas/Person'
      car: '#/components/schemas/Vehicle'

would previously generate an enum like

enum Foo {
  #[serde(rename="student")]
  Person { ... }
  #[serde(rename="teacher")]
  Person { ... }
  #[serde(rename="car")]
  Vehicle { ... }
}

This is a compiler error because there are duplicate enum variants.

The fix in this PR changes the generation to add the tag as a suffix to the variant name. (only if there are duplicates detected)

enum Foo {
  #[serde(rename="student")]
  PersonStudent { ... }
  #[serde(rename="teacher")]
  PersonTeacher { ... }
  #[serde(rename="car")]
  Vehicle { ... }
}

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