fix: Prevent AttributeError for _get_tags_from_request_kwargs (#14735)

* fix: avoid NoneType AttributeError when extracting tags

I've been running into this error:
```
21:47:08 - LiteLLM:ERROR: litellm_logging.py:2396 - LiteLLM.LoggingError: [Non-Blocking] Exception occurred while success logging Traceback (most recent call last):

  File "/usr/lib/python3.13/site-packages/litellm/litellm_core_utils/litellm_logging.py", line 2312, in async_success_handler

    await callback.async_log_success_event(

    ...<6 lines>...

    )

  File "/usr/lib/python3.13/site-packages/litellm/router_strategy/budget_limiter.py", line 396, in async_log_success_event

    request_tags = _get_tags_from_request_kwargs(kwargs)

  File "/usr/lib/python3.13/site-packages/litellm/router_strategy/tag_based_routing.py", line 144, in _get_tags_from_request_kwargs

    return _metadata.get("tags", [])

           ^^^^^^^^^^^^^

AttributeError: 'NoneType' object has no attribute 'get' 
```

This makes the function more resilient without resorting to try catch.

* add tests

Signed-off-by: Max Falk <gmdfalk@gmail.com>

---------

Signed-off-by: Max Falk <gmdfalk@gmail.com>

litellm/router_strategy/tag_based_routing.py

@@ -136,10 +136,10 @@ def _get_tags_from_request_kwargs(
     if request_kwargs is None:
         return []
     if metadata_variable_name in request_kwargs:
-        metadata = request_kwargs[metadata_variable_name]
+        metadata = request_kwargs[metadata_variable_name] or {}
         return metadata.get("tags", [])
     elif "litellm_params" in request_kwargs:
-        litellm_params = request_kwargs["litellm_params"]
-        _metadata = litellm_params.get(metadata_variable_name, {})
+        litellm_params = request_kwargs["litellm_params"] or {}
+        _metadata = litellm_params.get(metadata_variable_name, {}) or {}
         return _metadata.get("tags", [])
     return []

tests/test_litellm/router_strategy/test_router_tag_routing.py

@@ -385,4 +385,50 @@ async def test_router_free_paid_tier_with_responses_api():
         response_extra_info = response._hidden_params
         print("response_extra_info: ", response_extra_info)
 
-        assert response_extra_info["model_id"] == "very-expensive-model"
+        assert response_extra_info["model_id"] == "very-expensive-model"
+
+def test_get_tags_from_request_kwargs_none():
+    from litellm.router_strategy.tag_based_routing import _get_tags_from_request_kwargs
+
+    # None request kwargs should safely return empty list
+    assert _get_tags_from_request_kwargs(None) == []
+
+
+def test_get_tags_from_request_kwargs_various_inputs():
+    from litellm.router_strategy.tag_based_routing import _get_tags_from_request_kwargs
+
+    # Direct "metadata" path
+    assert _get_tags_from_request_kwargs({"metadata": {"tags": ["free"]}}) == ["free"]
+    assert _get_tags_from_request_kwargs({"metadata": {"tags": []}}) == []
+    assert _get_tags_from_request_kwargs({"metadata": {"tags": None}}) == []
+    assert _get_tags_from_request_kwargs({"metadata": {}}) == []
+    assert _get_tags_from_request_kwargs({"metadata": None}) == []
+
+    # Indirect via "litellm_params" - metadata inside
+    assert (
+        _get_tags_from_request_kwargs(
+            {"litellm_params": {"metadata": {"tags": ["paid"]}}}
+        )
+        == ["paid"]
+    )
+    assert _get_tags_from_request_kwargs({"litellm_params": {"metadata": None}}) == []
+    assert _get_tags_from_request_kwargs({"litellm_params": {}}) == []
+
+    # Alternate metadata variable name: "litellm_metadata"
+    assert (
+        _get_tags_from_request_kwargs(
+            {"litellm_metadata": {"tags": ["alt"]}},
+            metadata_variable_name="litellm_metadata",
+        )
+        == ["alt"]
+    )
+    assert (
+        _get_tags_from_request_kwargs(
+            {"litellm_params": {"litellm_metadata": {"tags": ["nested-alt"]}}},
+            metadata_variable_name="litellm_metadata",
+        )
+        == ["nested-alt"]
+    )
+
+    # No relevant keys present
+    assert _get_tags_from_request_kwargs({"foo": "bar"}) == []