PIPE-3085: Reduce MS open overhead in `hif_applycal` weblog rendering
Reduce the cost of opening measurement sets in get_brightest_field()
by reusing a single MS handle across all (field, spw) iterations instead
of opening and closing the MS once per combination. On lazy-import data
sets, where the DATA column is ASDM-backed, this cuts the applycal
post-processing time significantly (observed ~50 min saving on a
2496-call dataset).
- renderer.py: open MS once with casa_tools.MSReader and pass handle
down to mstools.compute_mean_flux via the new ms_handle kwarg.
- mstools.py: add keyword-only ms_handle parameter to both
read_channel_averaged_data_from_ms and compute_mean_flux; call
openms.reset() before each reuse to clear prior selection state.
