[BUG] [Rust] Generated identifiers should never start with an underscore

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?

I saw this in an OpenAPI-generated Rust crate here.

_type instead of r#type (or, alternatively, the less eyesore like type_).

Names prefixed with an underscore carry special meaning in the Rust language spec.
The Rust compiler ignores them for warnings etc.
A leading underscore in the name tells the compiler "this variable/member/function/whatever is unused".

Description

I presume this happens if isReservedWord(name) is true and the escapePrefix fed to the enclosing sanitizeIdentifier() function is empty/null and allowRawIdentifiers is false.

Suggest a fix

Modify this line from:

if (this.keywordsThatDoNotSupportRawIdentifiers.contains(name) || !allowRawIdentifiers) {
    name = casingFunction.apply(escapePrefix + '_' + name);
} else {
    name = "r#" + name;
};

to:

if (!Strings.isNullOrEmpty(escapePrefix) && (this.keywordsThatDoNotSupportRawIdentifiers.contains(name) || !allowRawIdentifiers)) {
    name = casingFunction.apply(escapePrefix + '_' + name);
} else {
    name = "r#" + name;
};

Start at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractRustCodegen.java around line 213 and trace how reserved Rust identifiers are sanitized. Generate or inspect a Rust client containing the reported identifier, then confirm the output no longer begins with an underscore and uses the expected raw or suffixed form.