fix: filter RU/KZ stopwords from KB search so filler words can't false-match

search_kb_rows had no relevance floor: any exact-token hit, however
generic, scored above zero and could win as the top/only result. A
caller utterance as thin as a bare "да" (confirming the language) could
exact-match that same common word inside an unrelated FAQ article's
body and get returned as "the" answer, which then got read back
almost verbatim — this is what surfaced live as the AI unprompted
launching into a voucher-activation explanation right after the
customer confirmed Russian, having said nothing else.

tokenize_kb_text now drops a curated set of RU/KZ greetings,
confirmations, pronouns, and particles. A stopword-only query naturally
falls through to the existing "no query tokens -> no results" path
instead of returning a coincidental match; genuine single-content-word
queries (e.g. "ваучер") are unaffected. Applies to every channel that
calls _kb_search (voice, Telegram, WhatsApp), not just voice.

tests/test_kb_search.py

@@ -114,6 +114,61 @@ def test_search_kb_rows_prefers_title_and_tags_over_body_only_mentions():
     assert results[0].title == rows[0].title
 
 
+def test_search_kb_rows_ignores_stopword_only_query():
+    # A caller confirming the language ("да") should never surface an
+    # unrelated FAQ article just because that article's body happens to
+    # contain the word "да" somewhere in ordinary prose.
+    rows = [
+        _row(
+            row_id=1,
+            title="Активация ваучера",
+            body="Да, подтвердите СМС с номера 1414 командой 21*1.",
+            tags=["ваучер"],
+        ),
+    ]
+
+    assert search_kb_rows(rows, "да", limit=5) == []
+    assert search_kb_rows(rows, "Хорошо", limit=5) == []
+
+
+def test_search_kb_rows_still_matches_real_content_word_amid_fillers():
+    rows = [
+        _row(
+            row_id=1,
+            title="Активация ваучера",
+            body="Подтвердите СМС с номера 1414 командой 21*1.",
+            tags=["ваучер"],
+        ),
+        _row(
+            row_id=2,
+            title="График работы",
+            body="Филиалы работают с 9 до 18.",
+            tags=["график"],
+        ),
+    ]
+
+    results = search_kb_rows(rows, "да, у меня вопрос про ваучер", limit=5)
+
+    assert results
+    assert results[0].title == "Активация ваучера"
+
+
+def test_search_kb_rows_single_real_word_query_still_matches():
+    rows = [
+        _row(
+            row_id=1,
+            title="Активация ваучера",
+            body="Подтвердите СМС с номера 1414 командой 21*1.",
+            tags=["ваучер"],
+        ),
+    ]
+
+    results = search_kb_rows(rows, "ваучер", limit=5)
+
+    assert results
+    assert results[0].title == "Активация ваучера"
+
+
 def test_search_kb_rows_breaks_ties_by_newer_id():
     older = _row(
         row_id=10,