esp32 driver: flashing fails with port locked message.

[mangelajo]

Sometimes when working with ESP32 devices, specially when using an LLM to drive the connection, a serial port can remain open by another cli (j serial...) , if you try to flash it will fail because the port has been locked.

I suggest that we force close the port from the flash driver command, making sure that the stream will be canceled, and then proceed to flash.

[ambient-code]

Fix Plan

Root cause: When a serial stream (e.g. from j serial pipe or an LLM-driven CLI session) holds the serial port open, esptool cannot access it for flash/dump/erase/chip-info operations, resulting in a "port locked" error.

Fix (in PR #660):

1. PySerial driver (jumpstarter_driver_pyserial): Added tracking of the active asyncio StreamWriter via _active_writer, and a new exported close_serial() method that force-closes it to release the port.

2. Esp32Flasher driver (jumpstarter_driver_esp32): Added _release_serial_port() which calls close_serial() on the child serial driver. This is invoked at the start of _connect_esp(), so every esptool operation (flash, dump, erase, get_chip_info) automatically releases any locked port before proceeding.

PR: #660

[ambient-code]

Fix Plan

Root cause: When a serial stream is active (e.g. via j serial), the PySerial driver holds the port open. When flash(), dump(), erase(), or get_chip_info() is called, esptool.cmds.detect_chip() tries to open the same port and fails with a "port locked" error.

Fix: Force-close any active serial stream before esptool opens the port.

Changes:

1. PySerial driver — Added _active_writer tracking and an @export disconnect() method that force-closes the underlying serial transport, releasing the port.
2. ESP32 flasher driver — _connect_esp() now calls self._serial.disconnect() before esptool.cmds.detect_chip(), ensuring the port is always free.
3. Tests — Added tests verifying disconnect() is safe when no stream is active, and that the call order (disconnect before detect_chip) is correct.

PR: #661