test: roll back runtime model registrations between tests

Since #35491, register_model records every registration in the process-global
_runtime_registered_model_cost ledger, and every cost map swap replays that
ledger on top of the freshly adopted map. Under pytest-xdist, any earlier test
in the same worker that registered gpt-3.5-turbo leaked into
TestPriceDataReloadIntegration::test_distributed_reload_check_function: the
replay ballooned its sparse mocked entry into a full ModelInfo dict and failed
the exact-equality assert, breaking the proxy-infra shard whenever loadscope
happened to co-schedule such a test first (reruns cannot help since the
pollution is process-wide)

The autouse isolate_litellm_state fixture now snapshots the ledger before each
test and restores it in place on teardown, so no test's registrations outlive
it. A regression pair in test_conftest_isolation.py asserts the rollback

tests/test_litellm/conftest.py

@@ -19,6 +19,7 @@ sys.path.insert(
 import asyncio
 
 import litellm
+from litellm import utils as litellm_utils_module
 from litellm._logging import ALL_LOGGERS
 from litellm.litellm_core_utils.prompt_templates import (
     image_handling as image_handling_module,
@@ -238,6 +239,11 @@ def isolate_litellm_state():
         if hasattr(litellm, _attr):
             original_state[_attr] = getattr(litellm, _attr)
 
+    original_runtime_registered_model_cost = {
+        model_key: dict(model_value)
+        for model_key, model_value in litellm_utils_module._runtime_registered_model_cost.items()
+    }
+
     # Store LiteLLM logger state. Some tests reconfigure handlers/propagation for
     # JSON logging and do not restore them, which breaks later caplog-based tests.
     logger_state = {}
@@ -304,6 +310,9 @@ def isolate_litellm_state():
         if hasattr(litellm, attr_name):
             setattr(litellm, attr_name, original_value)
 
+    litellm_utils_module._runtime_registered_model_cost.clear()
+    litellm_utils_module._runtime_registered_model_cost.update(original_runtime_registered_model_cost)
+
     # Restore logger configuration mutated by logging-focused tests.
     for logger in ALL_LOGGERS:
         original_logger_state = logger_state.get(logger.name)

tests/test_litellm/test_conftest_isolation.py

@@ -0,0 +1,13 @@
+import litellm
+from litellm import utils as litellm_utils_module
+
+CANARY_MODEL = "conftest-isolation-canary-model"
+
+
+def test_register_model_ledger_entry_is_scoped_to_this_test():
+    litellm.register_model({CANARY_MODEL: {"litellm_provider": "openai", "input_cost_per_token": 0.001}})
+    assert CANARY_MODEL in litellm_utils_module._runtime_registered_model_cost
+
+
+def test_register_model_ledger_entry_was_rolled_back():
+    assert CANARY_MODEL not in litellm_utils_module._runtime_registered_model_cost