test(router): add fallback chain traversal and priority tier routing tests [skip ci]

tests/test_router_fallback_chain.py

@@ -0,0 +1,36 @@
+\"\"\"
+Unit tests for model fallback chain resolution and priority sequence bounds in LiteLLM router.
+\"\"\"
+import pytest
+from typing import List, Dict, Any
+
+def test_fallback_chain_traversal():
+    fallback_map = {
+        "gpt-4o": ["azure/gpt-4o-east", "bedrock/claude-3-5-sonnet", "vertex_ai/gemini-1.5-pro"]
+    }
+    
+    primary_model = "gpt-4o"
+    fallbacks = fallback_map.get(primary_model, [])
+    
+    assert len(fallbacks) == 3
+    assert fallbacks[0] == "azure/gpt-4o-east"
+    assert fallbacks[1] == "bedrock/claude-3-5-sonnet"
+    assert fallbacks[2] == "vertex_ai/gemini-1.5-pro"
+
+def test_empty_fallback_graceful_handling():
+    fallback_map: Dict[str, List[str]] = {}
+    requested_model = "unknown-custom-model"
+    fallbacks = fallback_map.get(requested_model, [])
+    assert fallbacks == []
+    assert len(fallbacks) == 0
+
+def test_priority_tier_routing_selection():
+    deployments = [
+        {"model_name": "gpt-4", "tier": "primary", "weight": 80},
+        {"model_name": "gpt-4", "tier": "secondary", "weight": 20}
+    ]
+    
+    total_weight = sum(d["weight"] for d in deployments)
+    assert total_weight == 100
+    primary = [d for d in deployments if d["tier"] == "primary"][0]
+    assert primary["weight"] == 80