[BUG] Duplicate access token headers in Rust Client

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• [ x ] Have you validated the input using an OpenAPI validator (example)?
• [ ] Have you tested with the latest master to confirm the issue still exists?
• [ x ] Have you searched for related issues/PRs?
• [ x ] What's the actual output vs expected output?
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Description

Using the Rust Client generator, the template is causing the authentication failures using the oauth token.

The errors are caused by duplicate headers added to the requests:

    if let Some(ref local_var_token) = local_var_configuration.oauth_access_token {
        local_var_req_builder = local_var_req_builder.bearer_auth(local_var_token.to_owned());
    };
    if let Some(ref local_var_token) = local_var_configuration.oauth_access_token {
        local_var_req_builder = local_var_req_builder.bearer_auth(local_var_token.to_owned());
    };

openapi-generator version

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/12163
https://github.com/OpenAPITools/openapi-generator/issues/11318

Suggest a fix

I haven't figured out how to properly build with custom templates, but my suspicion is that the template code is the issue, with both the bearer and oauth options point to the same header call.

https://github.com/OpenAPITools/openapi-generator/blob/12fd115af3d72ff5cef59dacc6fab3f9d4c47b7e/modules/openapi-generator/src/main/resources/rust/reqwest/api.mustache#L218

Start by inspecting modules/openapi-generator/src/main/resources/rust/reqwest/api.mustache around line 218, then generate a Rust client with an OAuth access token and compare the emitted request headers. Done means the generated request adds the access-token authentication header only once; related issues 12163 and 11318 may provide context.