…te parsing

- add cleanup_old_processes() to proactively kill lingering chromedriver via pkill
- call cleanup_old_processes() at start of perform_checks() to prevent orphaned drivers
- enhance run_wifi_diagnostics_task(): detect Tx rate across multiple wdutil keys
  (Tx Rate, TxRate, Last Tx Rate, Max PHY Rate)
- keep robust managed_webdriver_session post-run cleanup as secondary safeguard

Tests
- test_cleanup.py: cover pkill invocation and exception handling
- test_main_flow.py: mock cleanup_old_processes; assert it runs first in perform_checks
- test_core.py: parametrize Wi‑Fi Tx rate key variants; maintain existing success/failure cases
- minor import/lint cleanups in tests

Notes
- aligns with acceptance criteria: pre-emptive cleanup logs, no orphaned chromedriver, primary flows intact

Refs
Ticket: Implement Pre-emptive and Post-run Process Cleanup