Imported CostMetadata into fallback_selector.py to access per-plan budget data.
Extended FallbackSelector.__init__ with cost_metadata: CostMetadata | None = None and stored it in self._cost_metadata.
Implemented per-plan budget check in FallbackSelector.select() immediately after the daily budget validation to enforce per-plan limits during selection.
Added two new TDD scenarios to features/cost_controls.feature to exercise per-plan budget behavior, tagged @tdd_issue @tdd_issue_10471.
Added new test steps at features/steps/tdd_fallback_plan_budget_steps.py to support the new scenarios.
ISSUES CLOSED: #10485