fix: Fix buffering issues with CP2102 converter causing connection failures

Some CP2102 drivers on Windows don't properly flush buffers after
unsupported commands, leaving stale data that breaks subsequent
communication. Add active buffer draining to read and discard any
lingering data before continuing.

Closes #1106

Author: Dzarda7

esptool/loader.py

@@ -502,10 +502,35 @@ def command(
                 if op is None or op_ret == op:
                     return val, data
                 if byte(data, 0) != 0 and byte(data, 1) == self.ROM_INVALID_RECV_MSG:
-                    # Unsupported read_reg can result in
+
+                    def drain_input_buffer(buffering_time=0.2):
+                        """
+                        Actively drain the input buffer by reading data
+                        for a specified time. Simple approach for some
+                        drivers that have issues with the buffer flushing.
+
+                        Args:
+                            buffering_time: Time in seconds to wait for
+                            the buffer to fill.
+                        """
+                        time.sleep(buffering_time)
+                        original_timeout = self._port.timeout
+                        # Set a very short timeout for draining
+                        self._port.timeout = 0.001
+
+                        # Unsupported command response is sent 8 times and has
+                        # 14 bytes length including delimiter 0xC0 bytes.
+                        # At least part of it is read as a command response,
+                        # but to be safe, read it all.
+                        self._port.read(14 * 8)
+
+                        # Restore original timeout
+                        self._port.timeout = original_timeout
+                        self.flush_input()
+
+                    # Unsupported command can result in
                     # more than one error response for some reason
-                    time.sleep(0.2)  # Wait for input buffer to fill
-                    self.flush_input()  # Flush input buffer of hanging response
+                    drain_input_buffer(0.2)
                     raise UnsupportedCommandError(self, op)
 
         finally: