feat(prompt-caching): add comments clarifying cache miss and partial match behavior

Add comment explaining cache miss must leave all healthy deployments eligible. Add comment clarifying only exact cached-prefix matches establish deployment affinity while partial matches fall back to normal routing.

litellm/router_utils/pre_call_checks/prompt_caching_deployment_check.py

@@ -98,6 +98,7 @@ class PromptCachingDeploymentCheck(CustomLogger):
                 request_kwargs.get("system") if request_kwargs is not None else None,
             )
 
+            # A cache miss must leave all healthy deployments eligible.
             model_id_dict: Final = await prompt_cache.async_get_model_id(
                 messages=affinity_messages,
                 tools=(

litellm/router_utils/prompt_caching_cache.py

@@ -217,6 +217,8 @@ class PromptCachingCache:
         if cacheable_messages is not None and not cacheable_messages:
             return None
 
+        # Only exact cached-prefix matches can establish deployment affinity.
+        # Partial matches must fall back to normal routing.
         # Use serialize_object for consistent and stable serialization
         data_to_hash: Final = {}
         if cacheable_messages is not None: