PIPE-3239: Add bpsolint_mode parameter to VLA bandpass tasks
Add task parameter `bpsolint_mode` to `hifv_testBPdcals`, `hifv_semiFinalBPdcals`,
and `hifv_finalcals` to control bandpass spectral solint optimization heuristics.
This prevents spectral averaging from corrupting bandpass solutions at lower
frequencies (X-band and below) where low S/N is frequently caused by RFI rather
than true low signal.
Supported modes:
- 'auto' (default): optimize only for Ku-band and higher frequencies (>= 12 GHz)
- 'on': optimize for all bands
- 'off': disable optimization for all bands
