Raise error on coord/variable name collisions

[Chirag3841]

Description

Related Issue

• Closes BUG: Missing check for collision between coord name and data variable name #7788
• [ x] Related to BUG: Missing check for collision between coord name and data variable name #7788

Checklist

• [x ] Checked that the pre-commit linting/style checks pass
• [x ] 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
• [ x] Bug fix
• Documentation
• Maintenance
• Other (please specify):

Description

It adds explicit validation checks in:
Model.add_coord: raises if a coord name already exists in Model.named_vars
Model.add_named_variable: raises if a variable name already exists in Model.coords
This ensures invalid models fail early and avoids silent downstream issues .

[welcome]

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[Chirag3841]

@drbenvincent
I have implemented the changes and added tests too. Please have a look.