Fix terminal echo race condition in monitor for character-by-character input

[Copilot]

Interactive applications like dmell that read input character-by-character were experiencing duplicate/interleaved characters (typing "ls" → "sls", "echo" → "ecoho") due to a race condition in the monitor's terminal configuration.

Root Cause

The monitor was restoring terminal echo to ON after reading each character. When firmware reads input character-by-character with echo disabled for manual control (using Dmod_Stdin_SetFlags), the sequence caused:

1. Monitor configures terminal: echo=OFF (correct)
2. User types character, monitor sends to firmware
3. Monitor restores terminal: echo=ON (race condition!)
4. User types next character while echo is ON → terminal echoes it
5. Firmware also manually echoes the character via dmlog output
6. Result: Characters appear twice, duplicated/interleaved

Changes

• tools/monitor/monitor.c:

  • Remove eager terminal restoration after each character read
  • Terminal configuration now persists between reads and is only reconfigured on next input request based on firmware's flags
  • Added monitor_restore_terminal() function with error checking for proper cleanup
  • Fixed O_NONBLOCK flag not being cleared when switching to line mode
  • Terminal properly restored only on monitor exit or error paths

• tools/monitor/main.c:

  • Added signal handlers (SIGINT/SIGTERM) for graceful terminal restoration
  • Call monitor_restore_terminal() on normal exit

• tools/monitor/monitor.h:

  • Added monitor_restore_terminal() function declaration

// Before: Terminal eagerly restored to echo=ON after each character
configure_input_mode(echo_on, line_mode);
// ... read character ...
configure_input_mode(true, true); // BUG: Re-enables echo while user still typing!

// After: Terminal settings persist between character reads
configure_input_mode(echo_on, line_mode);
// ... read character ...
// No restoration - next input request will configure as needed

Testing

• ✅ All unit tests pass (5/5)
• ✅ Build succeeds with no warnings
• ✅ Code review and security scan passed
• ✅ Correctly supports both echo enabled and disabled modes as controlled by firmware via Dmod_Stdin_SetFlags

The monitor now properly respects the DMLOG_FLAG_INPUT_ECHO_OFF flag without causing race conditions during character-by-character input.

Original prompt

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This section details on the original issue you should resolve

<issue_title>rozwiazac problem z echo</issue_title>
<issue_description>Jest jakis problem z echo z monitorem - nie jestem pewien gdzie, ale uzywanie jest tutaj:

https://github.com/choco-technologies/dmell/blob/main/src/dmell_ia.c

Jak odpalamy te apke na x86 (bez dmlog) to wszystko dziala poprawnie, natomiast jak uzywamy jej w dmod-boot (z dmlog) to mamy jako efekt cos takiego:

stm32f746xg@/>       
stm32f746xg@/> sls   
[WARN] Could not create mutex

stm32f746xg@/> ls etest
[ERROR] Failed to open directory 'test'
[ERROR] Failed to open directory 'test'
stm32f746xg@/> sls

stm32f746xg@/> sls -

stm32f746xg@/> ecoho "etest"
test
stm32f746xg@/> 

Wpisywałem po kolei:

ls
ls test
ls -
echo "test"
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes rozwiazac problem z echo #52

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[JohnAmadis]

@copilot to nie jest prawdą. Firmware ustawia flagi echo za pomocą interfejsu Dmod_Stdin_SetFlags i aplikacja dmell czyści flagę echo w dmlog a ten w monitorze. Musimy wspierać zarówno włączone echo jak i nie. Nie wiem czy to jest bug w monitorze czy w dmlog ale masz znaleźć root cause

[Copilot]

@copilot to nie jest prawdą. Firmware ustawia flagi echo za pomocą interfejsu Dmod_Stdin_SetFlags i aplikacja dmell czyści flagę echo w dmlog a ten w monitorze. Musimy wspierać zarówno włączon...

You were right - my initial fix was incorrect. The root cause was a race condition in terminal configuration.

The monitor was restoring terminal echo to ON after each character read. When firmware reads character-by-character with echo disabled (for manual control), this caused:

1. Terminal echoes next character (because echo was re-enabled)
2. Firmware manually echoes same character via dmlog output
3. Result: duplicate/interleaved characters (e.g., "ls" → "sls")

Fixed in commits 7138483 and d1c7d1c:

• Don't restore terminal settings between character reads
• Let next input request configure terminal based on firmware flags
• Only restore on monitor exit/error
• Added proper cleanup with signal handlers
• Fixed O_NONBLOCK not being cleared in line mode

The monitor now correctly supports both echo enabled and disabled modes as you required.