fix(serialport): data race on the isClosing bool status of the serial port

[dido18]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?
  Fix a DATA RACE on the isClosing variable of the seiral port.

• What is the current behavior?
  There are two routines that read and write the isClosing variable concurrently.
  • the routine of the open command reads the isClosing variable to verify if it must stop reading from the serial port
  • the routine of the close command writes the isClosing (setting to true) variable to inform that the serial port must be closed.

Steps to reproduce

• connect a board to the PC
• launch the cloud-agent locally with task run
• visit the debug console http://127.0.0.1:8991/
• type open /dev/ttyACM0 115200
• type close /dev/ttyACM0
• the DATA RACE warning appears in the console where the cloud-agent is running:

2025/01/20 10:12:07 stderr: ==================
2025/01/20 10:12:07 stderr: WARNING: DATA RACE
2025/01/20 10:12:07 stderr: Read at 0x00c0002b8b18 by goroutine 61:
2025/01/20 10:12:07 stderr:   main.(*serport).reader()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/serialport.go:88 +0x167
2025/01/20 10:12:07 stderr:   main.spHandlerOpen()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/serialport.go:345 +0x1467
2025/01/20 10:12:07 stderr:   main.checkCmd.gowrap4()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/hub.go:157 +0x6b
2025/01/20 10:12:07 stderr: 
2025/01/20 10:12:07 stderr: Previous write at 0x00c0002b8b18 by goroutine 66:
2025/01/20 10:12:07 stderr:   main.(*serport).Close()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/serialport.go:351 +0x37
2025/01/20 10:12:07 stderr:   main.spClose()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/serial.go:267 +0xee
2025/01/20 10:12:07 stderr:   main.checkCmd.gowrap5()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/hub.go:163 +0x44
2025/01/20 10:12:07 stderr: 
2025/01/20 10:12:07 stderr: Goroutine 61 (running) created at:
2025/01/20 10:12:07 stderr:   main.checkCmd()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/hub.go:157 +0x366
2025/01/20 10:12:07 stderr:   main.(*hub).run()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/hub.go:111 +0x244
2025/01/20 10:12:07 stderr:   main.loop.gowrap2()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/main.go:412 +0x33
2025/01/20 10:12:07 stderr: 
2025/01/20 10:12:07 stderr: Goroutine 66 (running) created at:
2025/01/20 10:12:07 stderr:   main.checkCmd()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/hub.go:163 +0x584
2025/01/20 10:12:07 stderr:   main.(*hub).run()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/hub.go:111 +0x244
2025/01/20 10:12:07 stderr:   main.loop.gowrap2()
2025/01/20 10:12:07 stderr:       /home/dido/code/arduino/arduino-create-agent/main.go:412 +0x33
2025/01/20 10:12:07 stderr: ==================

• What is the new behavior?
  Add an atomic.Bool to control the access of the isClosing status of the serial port.

• Does this PR introduce a breaking change?

• Other information:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 20.14%. Comparing base (67db428) to head (567c865).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
serialport.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1009      +/-   ##
==========================================
- Coverage   20.14%   20.14%   -0.01%     
==========================================
  Files          42       42              
  Lines        3221     3222       +1     
==========================================
  Hits          649      649              
- Misses       2487     2488       +1     
  Partials       85       85              

Flag	Coverage Δ
unit	20.14% <0.00%> (-0.01%)	⬇️

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[cmaglie]

If we only need to synchronize access to a single variable maybe an atomic.Bool could be better?

[cmaglie]

Retested on Linux, and now seems working OK! ✅