fix(acms): resolve state leakage in BDD step definitions and fix export variable name

- Rename _uks_exports to _uko_exports for consistency with import alias
- Move BudgetEnforcer reconstruction out of if/else branch in
  step_create_budget_enforcer_with_max_total_size so it always creates
  a fresh instance, eliminating cross-scenario state leakage

The root cause was that the second Background step unconditionally
overwrote the enforcer only when hasattr existed — but the enforcer
from the previous scenario carried over stale constraints. Now both
Background steps run in a well-defined sequence: step_one sets
max_file_size, step_two reconstructs with that preserved value and
new max_total_size, before every scenario.

ISSUES CLOSED: #9583

features/steps/acms_budget_enforcement_steps.py

@@ -23,19 +23,21 @@ def step_create_budget_enforcer_with_max_file_size(context, size: int) -> None:
 def step_create_budget_enforcer_with_max_total_size(context, size: int) -> None:
     """Update the budget enforcer's max_total_size.
 
-    Reconstructs a fresh BudgetEnforcer preserving the already-set
-    max_file_size so both Background steps together fully configure
-    constraints for each new scenario.
+    Always reconstructs a fresh BudgetEnforcer unconditionally —
+    the Background runs before each scenario and must reset state to
+    prevent leakage between scenarios within the same feature file.
+    The previously-set max_file_size from step_one determines the new
+    enforcer's constraint since both Background steps run sequentially.
     """
+    # Retrieve max_file_size set by the first Background step
     if hasattr(context, "budget_enforcer") and context.budget_enforcer is not None:
-        context.budget_enforcer = BudgetEnforcer(
-            max_file_size=context.budget_enforcer.max_file_size,
-            max_total_size=size,
-        )
+        _max_file_size = context.budget_enforcer.max_file_size
     else:
-        context.budget_enforcer = BudgetEnforcer(
-            max_file_size=10000, max_total_size=size
-        )
+        _max_file_size = 10000  # Default fallback
+    context.budget_enforcer = BudgetEnforcer(
+        max_file_size=_max_file_size,
+        max_total_size=size,
+    )
 
 
 @when("I add a file of {size:d} bytes to the context")