Basic Authorization Header is not correctly encoded in requests to OIDC servers

Q&A (please complete the following information)

• OS: Windows 10
• Browser: Edge
• Version: 102
• Method of installation: Swashbuckle (ASP.NET Core)
• Swagger-UI version: 4.10.3
• Swagger/OpenAPI version: OpenAPI 3.0.1

Content & configuration

    "securitySchemes": {
      "oidc": {
        "type": "openIdConnect",
        "openIdConnectUrl": "https://example.com/.well-known/openid-configuration"
      }
    }

Describe the bug you're encountering

When using a ClientId or ClientSecret that contains non ASCII characters, then a compliant OIDC server will reject the Authorization Header from swagger-ui due to the clientid / clientsecret not being application/x-www-form-urlencoded, trying to interpret the unicode characters as ASCII. https://datatracker.ietf.org/doc/html/rfc6749#section-2.3.1

To reproduce...

1. Open swagger
2. Authenticate via client_credentials using a ClientSecret that contains § for example.
3. OIDC server will respond with "invalid client credentials"

Expected behavior

If the OpenAPI specification states an openIdConnectUrl that specifies client_secret_basic, then the basic credentials should be application/x-www-form-urlencoded prior to base64 encoding. For OAuth2 servers, the most compatible option is probably the current behavior.

No source file or test is named in the issue. Start by tracing Swagger UI's OpenID Connect client-credentials handling for the securitySchemes configuration, then compare Authorization-header encoding with RFC 6749 §2.3.1; done means the reported non-ASCII client secret works for OIDC client_secret_basic without changing OAuth2 behavior.