PIPE-3250: Aggregate multi-MS reference frequencies for fracbw calculation in auto_selfcal
- In selfcal_helpers.get_mean_freq, aggregate reference frequencies across all MSes in vislist rather than querying vislist[0] alone.
- Resolves underestimation of fractional bandwidth (fracbw) when input visibilities are partitioned into per-SPW SubMS slices, ensuring correct nterms selection in get_nterms.
- Use casa_tools.TableReader context manager for safe table access, normalize spwsarray inputs across formats, and guard against out-of-bounds SPW indices and zero-division.
