test(vertex_ai): add e2e tests for rerank userLabels propagation

Cover the full litellm.rerank()/arerank() path with HTTP mocked, asserting
metadata.requester_metadata reaches the Discovery Engine :rank body as
userLabels (and stays absent when no metadata is set). Catches plumbing
regressions that unit tests on transform_rerank_request alone would miss.

tests/test_litellm/llms/vertex_ai/rerank/test_vertex_ai_rerank_userlabels_e2e.py

@@ -0,0 +1,182 @@
+"""
+End-to-end tests for Vertex AI rerank `userLabels` propagation.
+
+These tests go through the full `litellm.rerank()` call path with the HTTP
+layer mocked, so they catch plumbing bugs (e.g. `litellm_params` losing
+`metadata` between the rerank entrypoint and the Vertex transform) that
+unit tests on `VertexAIRerankConfig.transform_rerank_request` miss.
+"""
+
+import asyncio
+import json
+import os
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import httpx
+import pytest
+
+import litellm
+import litellm.llms.vertex_ai.rerank.transformation
+
+
+def _extract_body(call_kwargs):
+    """The rerank handler sends `data=json.dumps(...)`, not `json=...`."""
+    if "json" in call_kwargs and call_kwargs["json"] is not None:
+        return call_kwargs["json"]
+    raw = call_kwargs.get("data")
+    if isinstance(raw, (bytes, bytearray)):
+        raw = raw.decode("utf-8")
+    return json.loads(raw) if raw else None
+
+
+def _make_mock_rank_response():
+    mock_response = MagicMock(spec=httpx.Response)
+    mock_response.status_code = 200
+    mock_response.headers = {"content-type": "application/json"}
+    mock_response.json.return_value = {
+        "records": [
+            {"id": "0", "score": 0.9, "title": "doc 0", "content": "hello"},
+            {"id": "1", "score": 0.1, "title": "doc 1", "content": "world"},
+        ]
+    }
+    mock_response.text = '{"records": []}'
+    return mock_response
+
+
+def _make_async_mock_rank_response():
+    mock_response = AsyncMock()
+    mock_response.status_code = 200
+    mock_response.headers = {"content-type": "application/json"}
+    mock_response.json = MagicMock(
+        return_value={
+            "records": [
+                {"id": "0", "score": 0.9, "title": "doc 0", "content": "hello"},
+            ]
+        }
+    )
+    mock_response.text = '{"records": []}'
+    return mock_response
+
+
+@pytest.fixture
+def clean_vertex_env():
+    saved = {}
+    for var in (
+        "GOOGLE_APPLICATION_CREDENTIALS",
+        "GOOGLE_CLOUD_PROJECT",
+        "VERTEXAI_PROJECT",
+        "VERTEXAI_CREDENTIALS",
+        "VERTEX_AI_CREDENTIALS",
+        "VERTEX_PROJECT",
+        "VERTEX_LOCATION",
+        "VERTEX_AI_PROJECT",
+    ):
+        if var in os.environ:
+            saved[var] = os.environ.pop(var)
+    yield
+    for var, value in saved.items():
+        os.environ[var] = value
+
+
+def _patch_vertex_auth():
+    return patch.object(
+        litellm.llms.vertex_ai.rerank.transformation.VertexAIRerankConfig,
+        "_ensure_access_token",
+        return_value=("test-access-token", "test-project-2049"),
+    )
+
+
+def test_rerank_userlabels_propagates_from_metadata_sync(clean_vertex_env):
+    """
+    `litellm.rerank(metadata={"requester_metadata": {...}})` must end up as
+    `userLabels` on the Discovery Engine `:rank` request body.
+    """
+    captured = {}
+
+    def fake_post(*args, **kwargs):
+        captured["body"] = _extract_body(kwargs)
+        return _make_mock_rank_response()
+
+    with (
+        _patch_vertex_auth(),
+        patch(
+            "litellm.llms.custom_httpx.http_handler.HTTPHandler.post",
+            side_effect=fake_post,
+        ),
+    ):
+        litellm.rerank(
+            model="vertex_ai/semantic-ranker-default@latest",
+            query="what is gemini?",
+            documents=["hello", "world"],
+            vertex_project="test-project-2049",
+            vertex_credentials='{"type": "service_account"}',
+            metadata={"requester_metadata": {"team": "platform", "env": "prod"}},
+        )
+
+    body = captured["body"]
+    assert body is not None, "expected POST body to be captured"
+    assert "userLabels" in body, (
+        "Vertex rerank request body is missing `userLabels` — metadata was "
+        "lost between litellm.rerank() and transform_rerank_request. "
+        f"body keys: {sorted(body.keys())}"
+    )
+    assert body["userLabels"] == {"team": "platform", "env": "prod"}
+
+
+def test_rerank_userlabels_propagates_from_metadata_async(clean_vertex_env):
+    """Same as the sync test, but through `litellm.arerank`."""
+    captured = {}
+
+    async def fake_post(*args, **kwargs):
+        captured["body"] = _extract_body(kwargs)
+        return _make_async_mock_rank_response()
+
+    with (
+        _patch_vertex_auth(),
+        patch(
+            "litellm.llms.custom_httpx.http_handler.AsyncHTTPHandler.post",
+            side_effect=fake_post,
+        ),
+    ):
+        asyncio.run(
+            litellm.arerank(
+                model="vertex_ai/semantic-ranker-default@latest",
+                query="what is gemini?",
+                documents=["hello", "world"],
+                vertex_project="test-project-2049",
+                vertex_credentials='{"type": "service_account"}',
+                metadata={"requester_metadata": {"team": "platform"}},
+            )
+        )
+
+    body = captured["body"]
+    assert body is not None
+    assert body.get("userLabels") == {"team": "platform"}
+
+
+def test_rerank_userlabels_absent_when_no_metadata(clean_vertex_env):
+    """No metadata → no `userLabels` key (don't send empty maps)."""
+    captured = {}
+
+    def fake_post(*args, **kwargs):
+        captured["body"] = _extract_body(kwargs)
+        return _make_mock_rank_response()
+
+    with (
+        _patch_vertex_auth(),
+        patch(
+            "litellm.llms.custom_httpx.http_handler.HTTPHandler.post",
+            side_effect=fake_post,
+        ),
+    ):
+        litellm.rerank(
+            model="vertex_ai/semantic-ranker-default@latest",
+            query="what is gemini?",
+            documents=["hello", "world"],
+            vertex_project="test-project-2049",
+            vertex_credentials='{"type": "service_account"}',
+        )
+
+    body = captured["body"]
+    assert body is not None
+    assert "userLabels" not in body