Warn on silent institution_id fallback in CMIP7 global attributes

[rbeucher]

Summary

• The institution_id -> institution global-attribute lookup in cmor-cvs.json is missing entries silently fell back to writing the raw ID, which fails wcrp_cmip7 [ATTR004]/[ATTR009] compliance checks with no indication of why.
• The cmip7-cmor-tables submodule pointer already carries an ACCESS-Consortium entry (picked up via a prior submodule bump), so this specific ID resolves correctly today — but the fallback stays dormant and silent for any other/renamed institution_id.
• _get_institution_name now emits a UserWarning when it falls back, so a missing CV entry surfaces as an actionable warning instead of six silent compliance failures downstream.

Test plan

• pixi run -e dev python -m pytest tests/unit/test_vocabulary_processors.py -q (113 passed)
• Updated test_get_institution_name_fallback_to_id to assert the warning fires

Fixes #621

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.0%. Comparing base (590f0aa) to head (19c0c03).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #636   +/-   ##
=====================================
  Coverage   77.0%   77.0%           
=====================================
  Files         40      40           
  Lines       8518    8519    +1     
  Branches    1593    1593           
=====================================
+ Hits        6562    6563    +1     
  Misses      1624    1624           
  Partials     332     332           

Flag	Coverage Δ
unit	77.0% <100.0%> (+<0.1%)	⬆️

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