gh-159340: Detect partial reductions in expected compiler warnings

[esadomer]

The improvement check only examines files with no remaining warnings, so a reduction from two warnings to one is missed. Compare the actual count for every finite ignore rule, including zero, while preserving wildcard rules.

Fixes #159340.

Validation (Windows debug build): both isolated tests passed (count boundaries and CLI partial improvements); reduced and zero-count cases failed before the fix. The isolated patch applies cleanly to the base revision.

• Issue: Detect partial reductions in expected compiler warnings #159340

Merge coordination: #159337 and #159339 add different regression tests to the same new test_check_warnings.py module. After the first PR merges, the remaining branches need to incorporate the landed tests before merging.