Investigate time contiguity failure for `tpt-u-hxy-u` variables in `ap8`

During diagnostic review testing of UKCM2-0-LL historical u-dx055 the following set of variables failed QC due to the error `"Total length of time coordinate, 29216, is different from 29217 implied by time bounds (from 1900-01-01T00:00:00Z to 1910-01-01T00:00:00Z) and time frequency (PT3H)"`. This should be investigated as it appears to prevent the production of all `tpt-u-hxy-u` type variables for this stream.

The affected variables here are:
`cltcalipso_tpt-u-hxy-u|rsds_tpt-u-hxy-u|rsdscs_tpt-u-hxy-u|rsut_tpt-u-hxy-u|clt_tpt-u-hxy-u|rldscs_tpt-u-hxy-u|clwvi_tpt-u-hxy-u|rsutcs_tpt-u-hxy-u|rlds_tpt-u-hxy-u|clivi_tpt-u-hxy-u`

Reproduce the QC failure for the affected `tpt-u-hxy-u` variables in the UKCM2-0-LL historical `u-dx055` stream, using the reported time bounds and PT3H frequency. Trace the time-coordinate validation and production path to determine why 29216 points are produced instead of 29217. Done means the cause is identified and the production/QC behavior is corrected or clearly documented with regression coverage.

Written by the indexing model from the issue text.