Improve time gap error messages with diagnostic detail

[rbeucher]

Summary

When time bounds continuity checks fail, the error message previously gave no indication of where in the time axis the problem was.

Before

ValueError: Time bounds are not contiguous. Missing or overlapping timesteps detected in the CMOR time axis.

After

ValueError: Time bounds are not contiguous. Missing or overlapping timesteps detected in the CMOR time axis.
  1 discontinuity(ies) found:
  [index 11→12] time 1990-11-16 → 1991-01-16: bound end=1990-12-01, next bound start=1991-01-01

For the frequency-fallback path, intervals now show the surrounding timestamps instead of just the day counts:

ValueError: Missing timesteps detected in time coordinate for expected 'monthly' cadence.
Invalid interval(s) (1 total): 1990-11-16 → 1991-01-16 (61.00 days)

Changes

• Add _describe_bounds_gaps() static helper that identifies discontinuous bound indices and formats them with surrounding time coordinate values (up to 5 shown)
• _time_bounds_have_gaps() interface unchanged — existing tests unaffected
• Frequency-fallback gap list is now tuples of (t_before, t_after, delta_days) so timestamps appear in the error message

[codecov]

Codecov Report

❌ Patch coverage is 75.75758% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.8%. Comparing base (d0815eb) to head (c0e2ffa).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/access_moppy/base.py	75.8%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #551     +/-   ##
=======================================
- Coverage   74.9%   74.8%   -0.0%     
=======================================
  Files         36      36             
  Lines       7156    7191     +35     
  Branches    1333    1340      +7     
=======================================
+ Hits        5357    5381     +24     
- Misses      1519    1525      +6     
- Partials     280     285      +5     

Flag	Coverage Δ
unit	74.8% <75.8%> (-<0.1%)	⬇️

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