Only add dim_length shared variable when adding a new coordinate

[lucianopaz]

Description

Only create a model's dim_length SharedVariable when adding a new coordinate.

Related Issue

• Closes BUG: Model.add_coord can add multiple SharedVariable dimension length with the same name #7808
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7809.org.readthedocs.build/en/7809/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.84%. Comparing base (360cb6e) to head (8efaec4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7809   +/-   ##
=======================================
  Coverage   92.84%   92.84%           
=======================================
  Files         107      107           
  Lines       18380    18381    +1     
=======================================
+ Hits        17065    17066    +1     
  Misses       1315     1315           

Files with missing lines	Coverage Δ
pymc/model/core.py	93.16% <100.00%> (+0.14%)	⬆️

... and 1 file with indirect coverage changes

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