reliability: guard PPP connection cleanup paths

[geraldo-netto]

Summary

Fix PPP connection crash paths by making connection cleanup and polling state safe before the RFCOMM fd or pppd process exists.

Changes

• Initialize PPP lifecycle fields in PPPConnection.__init__
• Make cleanup() idempotent and resilient to os.close failures
• Avoid polling pppd before the process is created
• Remove timeout / IO watch sources on socket error, read error, and modem-init timeout paths
• Add focused tests for cleanup, early polling, socket errors, read errors, and timeout cleanup

Testing

• python -m py_compile blueman/main/PPPConnection.py test/main/test_pppconnection.py
• git diff --check

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud