delete redundant height_0 in variables coordinates attrs

[rhaegar325]

Issue to be fixed

The CMORised output for tas, hurs, huss (Amon) has

tas:coordinates = "height_0 height" ;

but only height is actually written to the file. height_0 is a dangling
reference, flagged by WCRP compliance-checker:

[ATTR004] Variable 'tas' attribute 'coordinates' as-variable check
  Referenced variables not found: ['height_0']

Fix

Replace the merge with a token-aware, dataset-validated one in
src/access_moppy/base.py:

if var == self.cmor_name:
    existing_tokens = vdat.attrs.get("coordinates", "").split()
    valid_existing = [t for t in existing_tokens if t in self.ds.variables]
    merged = list(dict.fromkeys(valid_existing + list(aux_coords)))
    if merged:
        v.setncattr("coordinates", " ".join(merged))
    elif "coordinates" in v.ncattrs():
        v.delncattr("coordinates")

• Drops tokens not present in self.ds.variables (kills the dangling
  height_0).
• Deduplicates with dict.fromkeys (preserves order).
• Removes the attribute entirely when nothing valid remains, instead of
  leaving a dangling reference inherited from line 1047's attribute copy.

Scope of the Change

Single block in src/access_moppy/base.py. Behaviour identical to the
previous code in every scenario where the previous output was already correct;
strictly improves the cases that previously produced dangling or duplicate
tokens. No existing tests assert on the output coordinates attribute, so no
test changes are required (a regression test is recommended but not blocking).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.2%. Comparing base (34d58b9) to head (2539737).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #410     +/-   ##
=======================================
+ Coverage   75.1%   75.2%   +0.1%     
=======================================
  Files         28      28             
  Lines       5281    5282      +1     
  Branches     973     973             
=======================================
+ Hits        3966    3972      +6     
+ Misses      1091    1089      -2     
+ Partials     224     221      -3     

Flag	Coverage Δ
unit	75.2% <100.0%> (+0.1%)	⬆️

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