test(classifier): exact-count assertions for placeholder fixture; document SERVICE_FIELD_KEYS prefix semantics

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>

includes/class-classifier.php

@@ -46,7 +46,12 @@ final class Classifier
         'rambler.ru', 'gmail.com',
     ];
 
-    /** Service field keys excluded from placeholder count. */
+    /**
+     * Explicit service field keys excluded from placeholder count.
+     * Note: keys starting with `_wpcf7` are also skipped via prefix match
+     * (see count_placeholder_fields), so this list does not need to enumerate
+     * every `_wpcf7_*` variant — only non-prefixed service keys.
+     */
     private const SERVICE_FIELD_KEYS = [
         'acceptance-data',
         '_wpcf7', '_wpcf7_version', '_wpcf7_locale', '_wpcf7_unit_tag',