Drop dead stub enum, gate get_modem_pins error ack on client 1.16, clarify ack comment

esphome/components/api/api.proto

@@ -2816,9 +2816,9 @@ message SerialProxyRequest {
   SerialProxyRequestType type = 2;      // Request type
 }
 
-// Acknowledges a serial proxy operation (subscribe, unsubscribe, flush,
-// configure, or modem pin change). Sent since API 1.16; the type field
-// identifies which operation is being acknowledged.
+// Acknowledges a serial proxy operation; the type field identifies which
+// operation is being acknowledged. Flush has been acknowledged since the
+// message was introduced; all other acknowledgements are sent since API 1.16.
 message SerialProxyRequestResponse {
   option (id) = 147;
   option (source) = SOURCE_SERVER;

esphome/components/api/api_connection.cpp

@@ -1632,6 +1632,11 @@ void APIConnection::on_serial_proxy_get_modem_pins_request(const SerialProxyGetM
   resp.instance = msg.instance;
   if (msg.instance >= proxies.size()) {
     ESP_LOGW(TAG, "Serial proxy instance %" PRIu32 " out of range", msg.instance);
+    // Pre-1.16 clients do not read the status field and would take this error
+    // for a successful "both pins deasserted" answer; let them time out as before
+    if (!this->client_supports_api_version(1, 16)) {
+      return;
+    }
     resp.status = enums::SERIAL_PROXY_STATUS_INVALID_ARGUMENT;
   } else {
     resp.line_states = proxies[msg.instance]->get_modem_pins();

tests/benchmarks/stubs/esphome/components/serial_proxy/serial_proxy.h

@@ -13,15 +13,6 @@ namespace api {
 class APIConnection;
 }  // namespace api
 
-namespace uart {
-enum class UARTFlushResult : uint8_t {
-  UART_FLUSH_RESULT_SUCCESS,
-  UART_FLUSH_RESULT_ASSUMED_SUCCESS,
-  UART_FLUSH_RESULT_TIMEOUT,
-  UART_FLUSH_RESULT_FAILED,
-};
-}  // namespace uart
-
 namespace serial_proxy {
 
 enum class SerialProxyResult : uint8_t {