fix: OIDC provider validation log message (#1380)

Message was:

```
Custom OIDC provider "" not allowed
```

And now is:

```
Custom OIDC provider "kakao" not allowed
```

internal/api/token_oidc.go

@@ -90,7 +90,7 @@ func (p *IdTokenGrantParams) getProvider(ctx context.Context, config *conf.Globa
 		}
 
 		if !allowed {
-			return nil, nil, "", nil, badRequestError(fmt.Sprintf("Custom OIDC provider %q not allowed", p.Issuer))
+			return nil, nil, "", nil, badRequestError(fmt.Sprintf("Custom OIDC provider %q not allowed", p.Provider))
 		}
 	}