Clarify dims error message to reference as_xtensor

[AdishreeB]

Description

This PR clarifies an error message raised when variables are missing associated dims.
The message now explicitly references pytensor.xtensor.as_xtensor(..., dims=...),
making it clearer how users should attach dims to observed data and reducing
confusion about where as_xtensor is defined.

No behaviour changes are introduced; this PR only improves a user-facing error message

Related Issue

• Closes Clarify ImportError location for as_xtensor in dims error message #8060
• 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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.80%. Comparing base (a375fd1) to head (374aaaa).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8066      +/-   ##
==========================================
+ Coverage   88.76%   90.80%   +2.03%     
==========================================
  Files         116      121       +5     
  Lines       18971    19426     +455     
==========================================
+ Hits        16840    17639     +799     
+ Misses       2131     1787     -344     

Files with missing lines	Coverage Δ
pymc/dims/distributions/core.py	89.55% <ø> (ø)

... and 43 files with indirect coverage changes

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