feat(backend): add sellability reason codes and string price serialization to test seat preview

extend test seat preview with explicit sellability reason codes

serialize preview price amount as string for a stable API contract
improve diagnosis of non-sellable states for preview consumers

backend/app/schemas/test_mode.py

@@ -1,12 +1,10 @@
-from decimal import Decimal
-
 from pydantic import BaseModel
 
 
 class TestSeatPreviewResponse(BaseModel):
     scheme_id: str
     scheme_version_id: str
-    seat_id: str
+    seat_id: str | None
     element_id: str | None
     sector_id: str | None
     group_id: str | None
@@ -17,5 +15,7 @@ class TestSeatPreviewResponse(BaseModel):
     matched_rule_level: str | None
     matched_target_ref: str | None
     pricing_category_id: str | None
-    amount: Decimal | None
+    amount: str | None
     currency: str | None
+    reason_code: str
+    reason_message: str