Refactored stats_refitting.py for better robustness and readability

[rrawatt]

Description

Refactored the code

1. core logic is kept same.
2. extracted scale conversion into a helper function (_get_scale_value) to keep the main function cleaner
3. replaced np.argwhere with np.flatnonzero for simpler, 1D indexing of problematic observations
4. np.sqrt instead of raising to the power of 0.5 for computing the standard error.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog