Disable NAGL as a partial charge backend when OEToolkit is installed but not chosen as the registry backend.

[IAlibay]

Fixes #1760

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• Added a news entry, or the changes are not user-facing.
• Ran pre-commit by making a comment with pre-commit.ci autofix before requesting review.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[IAlibay]

pre-commit.ci autofix

[codecov]

Codecov Report

❌ Patch coverage is 71.18644% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.17%. Comparing base (face41c) to head (f62bc1a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tests/protocols/openmm_rfe/test_hybrid_top_slow.py	19.04%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1762      +/-   ##
==========================================
- Coverage   95.42%   93.17%   -2.25%     
==========================================
  Files         190      190              
  Lines       16802    16818      +16     
==========================================
- Hits        16033    15670     -363     
- Misses        769     1148     +379     

Flag	Coverage Δ
fast-tests	93.17% <71.18%> (?)
slow-tests	?

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[github-actions]

No API break detected ✅