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/api/routes/test_mode.py

@@ -30,12 +30,6 @@ async def preview_test_seat(
         seat_id=seat_id,
     )
 
-    if not seat.seat_id:
-        raise HTTPException(
-            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
-            detail="Невозможно построить preview: у места отсутствует seat_id",
-        )
-
     matched_rule_level = None
     matched_target_ref = None
     pricing_category_id = None
@@ -43,6 +37,27 @@ async def preview_test_seat(
     currency = None
     has_price = False
 
+    if not seat.seat_id:
+        return TestSeatPreviewResponse(
+            scheme_id=scheme.scheme_id,
+            scheme_version_id=version.scheme_version_id,
+            seat_id=seat.seat_id,
+            element_id=seat.element_id,
+            sector_id=seat.sector_id,
+            group_id=seat.group_id,
+            row_label=seat.row_label,
+            seat_number=seat.seat_number,
+            selectable=False,
+            has_price=False,
+            matched_rule_level=None,
+            matched_target_ref=None,
+            pricing_category_id=None,
+            amount=None,
+            currency=None,
+            reason_code="missing_seat_id",
+            reason_message="Seat is not sellable because seat_id is missing.",
+        )
+
     try:
         matched_rule_level, rule = await find_effective_price_rule(
             scheme_id=scheme.scheme_id,
@@ -52,7 +67,7 @@ async def preview_test_seat(
         )
         matched_target_ref = rule["target_ref"]
         pricing_category_id = rule["pricing_category_id"]
-        amount = rule["amount"]
+        amount = str(rule["amount"])
         currency = rule["currency"]
         has_price = True
     except HTTPException as exc:
@@ -66,15 +81,25 @@ async def preview_test_seat(
         )
         raise HTTPException(
             status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
-            detail=f"Не удалось построить preview: {exc.__class__.__name__}: {exc}",
+            detail={
+                "code": "test_preview_failed",
+                "message": f"Не удалось построить preview: {exc.__class__.__name__}: {exc}",
+            },
         )
 
+    if has_price:
+        reason_code = "ok"
+        reason_message = "Seat is sellable."
+    else:
+        reason_code = "no_price_rule"
+        reason_message = "Seat is not sellable because no effective price rule was found."
+
     return TestSeatPreviewResponse(
         scheme_id=scheme.scheme_id,
         scheme_version_id=version.scheme_version_id,
         seat_id=seat.seat_id,
         element_id=seat.element_id,
-        sector_id=seat.sector_id,
+        sector_id=seat.seat_id and seat.sector_id,
         group_id=seat.group_id,
         row_label=seat.row_label,
         seat_number=seat.seat_number,
@@ -85,4 +110,6 @@ async def preview_test_seat(
         pricing_category_id=pricing_category_id,
         amount=amount,
         currency=currency,
+        reason_code=reason_code,
+        reason_message=reason_message,
     )