test: add unit tests for PassiveBuzzerHelper

[sakshar2303]

Summary

Closes #358

Tests Added

PassiveBuzzerHelperTest.java in com.opensourcewithslu.outputdevices

• Defaults — verifies frequency=440 and duty cycle=50 on construction
• passiveBuzzerOn() — correct PWM args, custom values, min/max boundary
• passiveBuzzerOff() — single and multiple calls
• getFrequency() — default and manually updated value
• passiveBuzzTone() — on before off, uses default freq and DC
• toneIterator() — all 5 frequencies played in order, on/off per frequency
• piToneSequence() — correct frequency for each pi digit in order, ends with off()
• setFrequencies() — valid file, out-of-range skip, file not found
  (no throw), single frequency, exact boundary values (20 Hz and 20000 Hz)

All PWM interactions verified via Mockito mock. All tests pass.