test(backend): add guard test for tier multiplier enum completeness

Ensures every SubscriptionTier has an entry in _DEFAULT_TIER_MULTIPLIERS.
Matches the existing storage limit guard — both fail immediately if
someone adds a new tier without updating the mappings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

autogpt_platform/backend/backend/copilot/rate_limit_test.py

@@ -1608,6 +1608,15 @@ class TestWorkspaceStorageLimits:
             )
             assert TIER_WORKSPACE_STORAGE_MB[tier] > 0
 
+    def test_every_subscription_tier_has_rate_limit_multiplier(self):
+        """Adding a new SubscriptionTier without a rate limit multiplier should fail."""
+        for tier in SubscriptionTier:
+            assert tier in _DEFAULT_TIER_MULTIPLIERS, (
+                f"SubscriptionTier.{tier.name} has no entry in "
+                f"_DEFAULT_TIER_MULTIPLIERS — add one"
+            )
+            assert _DEFAULT_TIER_MULTIPLIERS[tier] > 0
+
     @pytest.mark.parametrize(
         "tier,expected_mb",
         [