API output without Persian diacritics or written ezafe ی (company decision); tutor answers too

plain_persian() strips kasra/fatha/damma/shadda/sukun/hamza-above and the written «ه‌ی» ezafe, keeping tanwin. Applied
at output, not in data: PlainPersianJSONRenderer runs one byte-level pass over every JSON response (~5 ms per MB
locally), and to_plain() cleans tutor answers including the stream. Rewriting titles in the DB/seed would have broken
question binding by title path and SeedRecord keys. Tests updated to expect the cleaned text; config/tests_plain_persian.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

rag/services/mathtext.py

@@ -256,8 +256,9 @@ def _to_plain(text: str) -> str:
     text = re.sub(r'[ \t]+\n', '\n', text)
     text = re.sub(r'\n{3,}', '\n\n', text)
     # S_n، a_1، x^2 و توانِ پیچیده که بالا رد شد: همان تبدیلِ seed/دیتابیس (یک رفتار در همه‌جا).
-    from content.plain_math import convert as _plain
-    return _plain(text).strip()
+    from content.plain_math import convert as _plain, plain_persian
+    # بی‌اعراب و بی «ی»ِ اضافه، مثلِ همه‌ی پاسخ‌های API (config/renderers.py)؛ پاسخِ استریمی از آن renderer رد نمی‌شود.
+    return plain_persian(_plain(text)).strip()
 
 
 class StreamSanitizer: