[client] Pass the login hint to GetOAuthFlow at construction

GetOAuthFlow was the only flow factory without a hint parameter, which
forced its callers to apply the hint afterwards through a local setter
interface and a type assertion. Give it the same constructor-style hint
as NewOAuthFlow and set the hint on the concrete flows before they are
handed out as the interface, so a flow is always complete when built
and the caller-side ordering constraint disappears.

An empty hint is a valid value meaning the IdP chooses the account, so
the flows set it unconditionally.

client/internal/auth/oauth.go

@@ -97,9 +97,7 @@ func authenticateWithPKCEFlow(ctx context.Context, config *profilemanager.Config
 		return nil, fmt.Errorf("getting pkce authorization flow info failed with error: %v", err)
 	}
 
-	if hint != "" {
-		pkceFlowInfo.SetLoginHint(hint)
-	}
+	pkceFlowInfo.SetLoginHint(hint)
 
 	return pkceFlowInfo, nil
 }
@@ -127,9 +125,7 @@ func authenticateWithDeviceCodeFlow(ctx context.Context, config *profilemanager.
 		}
 	}
 
-	if hint != "" {
-		deviceFlowInfo.SetLoginHint(hint)
-	}
+	deviceFlowInfo.SetLoginHint(hint)
 
 	return deviceFlowInfo, nil
 }