fix(providers): enforce per-plan budget in FallbackSelector.select()

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

features/cost_controls.feature

@@ -469,3 +469,22 @@ Feature: Cost controls and provider fallback
     When I create a FallbackSelector with the registry
     And I select a fallback provider requiring json mode
     Then the fallback result should skip for missing json mode
+
+  @unit @cost @tdd_issue @tdd_issue_10471
+  Scenario: FallbackSelector skips provider when per-plan budget exceeded
+    Given I create a ProviderRegistry with openai key for fallback
+    And I create a CostTracker with plan budget 0.0
+    And I create a CostMetadata with total cost 1.0
+    When I create a FallbackSelector with cost tracker and cost metadata
+    And I select a fallback provider
+    Then the fallback result provider should be None
+    And the fallback skipped should mention "plan budget exceeded"
+
+  @unit @cost @tdd_issue @tdd_issue_10471
+  Scenario: FallbackSelector selects provider when per-plan budget not exceeded
+    Given I create a ProviderRegistry with openai key for fallback
+    And I create a CostTracker with plan budget 10.0
+    And I create a CostMetadata with total cost 1.0
+    When I create a FallbackSelector with cost tracker and cost metadata
+    And I select a fallback provider
+    Then the fallback result provider type should be "openai"

features/steps/tdd_fallback_plan_budget_steps.py

@@ -0,0 +1,35 @@
+"""Step definitions for TDD issue #10471: FallbackSelector per-plan budget enforcement."""
+
+from __future__ import annotations
+
+from behave import given, when  # type: ignore[import-untyped]
+from behave.runner import Context  # type: ignore[import-untyped]
+
+
+@given("I create a CostMetadata with total cost {cost:f}")
+def step_create_cost_metadata_with_total_cost(context: Context, cost: float) -> None:
+    from cleveragents.domain.models.core.cost_metadata import CostMetadata
+
+    meta = CostMetadata()
+    if cost > 0:
+        meta.record_usage(
+            input_tokens=1000,
+            output_tokens=500,
+            cost=cost,
+            provider="openai",
+        )
+    context.fallback_cost_metadata = meta
+
+
+@when("I create a FallbackSelector with cost tracker and cost metadata")
+def step_create_fallback_with_tracker_and_metadata(context: Context) -> None:
+    from cleveragents.providers.fallback_selector import FallbackSelector
+
+    tracker = getattr(context, "fallback_cost_tracker", None) or getattr(
+        context, "cost_tracker", None
+    )
+    context.fallback_selector = FallbackSelector(
+        registry=context.fallback_registry,
+        cost_tracker=tracker,
+        cost_metadata=context.fallback_cost_metadata,
+    )

src/cleveragents/providers/fallback_selector.py

@@ -10,6 +10,7 @@ import logging
 from dataclasses import dataclass
 from typing import ClassVar
 
+from cleveragents.domain.models.core.cost_metadata import CostMetadata
 from cleveragents.providers.cost_tracker import BudgetStatus, CostTracker
 from cleveragents.providers.registry import (
     ProviderCapabilities,
@@ -76,6 +77,7 @@ class FallbackSelector:
         *,
         registry: ProviderRegistry,
         cost_tracker: CostTracker | None = None,
+        cost_metadata: CostMetadata | None = None,
         fallback_providers: list[str] | None = None,
     ) -> None:
         """Initialize the fallback selector.
@@ -100,6 +102,7 @@ class FallbackSelector:
                     )
         self._registry = registry
         self._cost_tracker = cost_tracker
+        self._cost_metadata = cost_metadata
         self._fallback_order = (
             [p.strip().lower() for p in fallback_providers]
             if fallback_providers
@@ -160,6 +163,14 @@ class FallbackSelector:
                     skipped.append((provider_name, "daily budget exceeded"))
                     continue
 
+                if self._cost_metadata is not None:
+                    plan_check = self._cost_tracker.check_plan_budget(
+                        self._cost_metadata
+                    )
+                    if plan_check.status == BudgetStatus.EXCEEDED:
+                        skipped.append((provider_name, "plan budget exceeded"))
+                        continue
+
             logger.info(
                 "Fallback selected provider: %s (skipped %d)",
                 provider_name,