perf: optimize conflict resolution algorithm in AnonymizerEngine

[Ray0907]

Change Description

• Replace O(n) list.remove() with O(1) set operations for tracking active indices
• Use pop() instead of remove() in whitespace merge since prev_result is always last
• Convert list comprehension to generator expression for short-circuit evaluation
• Pre-compile regex pattern to avoid repeated compilation

Issue reference

Fixes #XX

Checklist

• I have reviewed the contribution guidelines
• I have signed the CLA (if required)
• My code includes unit tests
• All unit tests and lint checks pass locally
• My PR contains documentation updates / additions if required

[Ray0907]

@microsoft-github-policy-service agree