chore: remove defensive stale-guest degrade in regenerate.py (T73.3)

Phase 2 T44 added a defensive degrade-to-1:1 here when
`chat.guest_bot_id` pointed at a deleted bot. T47 fixed the root cause:
`bot_reset` cascade-clears the column when the referenced bot is purged
(verified by tests/test_reset.py), so the guard was dead code.

No corresponding stale-guest test existed in tests/test_regenerate.py
to remove. The bot_reset cascade test in tests/test_reset.py already
covers the root-cause behavior.

chat/services/regenerate.py

@@ -114,13 +114,13 @@ async def regenerate_assistant_turn(
 
     # Phase 2: surface the guest (if any) so the prompt assembler and
     # downstream multi-entity passes see the same shape post_turn does.
+    # Phase 2 T47 made bot_reset cascade-clear ``chat.guest_bot_id`` when
+    # the referenced bot is purged (verified by tests/test_reset.py), so
+    # we trust the column here: it's either a valid bot id or NULL.
     guest_bot_id = chat.get("guest_bot_id")
-    guest_bot: dict | None = None
-    if guest_bot_id is not None:
-        guest_bot = get_bot(conn, guest_bot_id)
-        if guest_bot is None:
-            # Stale guest reference — degrade to single-bot regenerate.
-            guest_bot_id = None
+    guest_bot: dict | None = (
+        get_bot(conn, guest_bot_id) if guest_bot_id is not None else None
+    )
 
     # 1. Locate the original assistant_turn event.
     row = conn.execute(