Fix: Filter noise and refine regex in AST reference extraction

[pyob-bot]

Summary of Changes

• Updated the AST node processing logic to exclude common keywords (self, None, True, False) from potential_refs when parsing Python files.
• Adjusted the fallback regex pattern for Python code parsing to increase the minimum identifier length from 3 to 4 characters.

Technical Impact

• Reduces false positives in reference tracking by filtering out language-reserved keywords and common built-ins.
• Improves the precision of the fallback regex mechanism by enforcing a stricter character count, which helps mitigate noise when AST parsing fails and the system falls back to regex-based extraction.