(Re)Add warning for rank deficiency

[mattansb]

The note for rank deficiency was broken, this PR re-establishes it and upgrades it to a warning.

Closes #922

[gemini-code-assist]

Code Review

This pull request updates check_collinearity() to handle rank-deficient models and issue appropriate warnings. However, there are a few critical issues in the implementation: an undefined variable m is referenced when identifying NA columns, an incorrect logical comparison any(!keep_idx) < ncol(v) is used during matrix subsetting, and a leftover debugonce() statement in the test file needs to be removed to prevent CI test failures.

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