redraw the figure on follow-ups that only change the numbers; add a local fake OpenAI server

"What if 6 were 4?" has no shape word, so the diagram hint never went next to
it and the tutor would answer without a new figure. When the previous user
question in the conversation had one, a soft hint (redraw only if this
continues that problem) and the same catalog sections now go along.

tools/fake_openai.py stands in for GapGPT locally so the whole tutor path can be
exercised without credit: 12 scripted answers across grades 6-12 went through
streaming, block splitting, repair, storage, history and rendering in the real
chat screen.

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

rag/services/generation.py

@@ -176,9 +176,10 @@ _SLASH_READING = (
 
 
 def _user_prompt(question: str, contexts: list[str], focus: str = '',
-                 primary: bool = False) -> str:
+                 primary: bool = False, previous: str = '') -> str:
     asked = f'سوالِ دانش‌آموز: {question}'
-    shape_hint = diagram_mod.hint_for(f'{question}\n{focus}')
+    shape_hint = (diagram_mod.hint_for(f'{question}\n{focus}')
+                  or diagram_mod.follow_up_hint_for(question, previous))
     if shape_hint:
         asked = f'{asked}\n\n{shape_hint}'
     slash_rule = primary and '/' in question
@@ -250,8 +251,11 @@ def build_messages(question: str, context_chunks: list[dict] | None,
         text = (turn.get('text') or '').strip()
         if text:
             messages.append({'role': role, 'content': text[:_HISTORY_CHARS]})
+    # سوالِ قبلیِ کاربر: «اگه به‌جای ۶ بذاریم ۴ چی؟» کلمه‌ی شکل‌دار ندارد، ولی ادامه‌ی
+    # همان مسئله‌ی شکل‌دار است و شکلش باید با عددِ تازه دوباره کشیده شود.
+    previous = next((t.get('text') or '' for t in reversed(history or []) if t.get('role') == 'user'), '')
     messages.append({'role': 'user', 'content': _user_prompt(
-        question, contexts, focus, primary=grade_tier(grade_title) == 'primary')})
+        question, contexts, focus, primary=grade_tier(grade_title) == 'primary', previous=previous)})
     return messages