refactor(api): unify typed error contract across draft pricing and publish flows

standardize typed error responses across draft, pricing and publish endpoints

reduce contract drift between related flows
keep client-side handling more predictable and consistent

backend/app/services/publish_service.py

@@ -1,9 +1,8 @@
-from fastapi import HTTPException, status
-
 from app.repositories.audit import create_audit_event
 from app.repositories.scheme_version_pricing import replace_scheme_version_pricing_snapshot
 from app.services.draft_guard import get_current_draft_context
 from app.repositories.schemes import publish_scheme
+from app.services.api_errors import raise_conflict
 from app.services.scheme_validation import build_scheme_validation_report
 
 
@@ -23,9 +22,11 @@ async def publish_current_draft_scheme(
     )
 
     if not validation["summary"]["is_publishable"]:
-        raise HTTPException(
-            status_code=status.HTTP_409_CONFLICT,
-            detail="Scheme is not publishable in current state",
+        raise_conflict(
+            code="publish_validation_failed",
+            message="Scheme is not publishable in current state",
+            scheme_version_id=version.scheme_version_id,
+            validation_summary=validation["summary"],
         )
 
     snapshot = await replace_scheme_version_pricing_snapshot(