Fixes issues from code review in commit 249f195:
1. Tighten fixture assertions to exact counts (assertSame instead of assertGreaterThanOrEqual):
- test_count_placeholder_fields_screenshot_1_exact_count: expect 5 placeholders
- test_count_placeholder_fields_screenshot_2_exact_count: expect 5 placeholders
- test_count_placeholder_fields_screenshot_3_exact_count: expect 1 placeholder
2. Document SERVICE_FIELD_KEYS prefix-matching semantics in comment:
clarify that keys starting with _wpcf7 are skipped via strpos prefix match,
so explicit enumeration of every _wpcf7_* variant is unnecessary.
Test results: 75/75 green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>