test(proxy_behavior): scaffold session-scoped async ASGI client + liveness smoke

Slice 2 of the management-endpoints behavior-pinning effort. New top-level dir
tests/proxy_behavior/management/ outside every existing pytest glob.

conftest.py initialises the proxy app once per session against the DATABASE_URL
the harness boots Postgres at, wraps it in httpx.AsyncClient via in-process
ASGITransport. The one smoke test asserts /health/liveliness returns 200, which
exercises the full FastAPI middleware stack against a real app — no mocks.

Plan: https://www.notion.so/36643b8acdab8128a581ced0f6a4744d

tests/proxy_behavior/management/conftest.py

@@ -0,0 +1,55 @@
+"""Session-scoped async ASGI client for behavior-pinning tests.
+
+The proxy app is initialised once per pytest session against the real Postgres
+pointed at by ``DATABASE_URL``. No mocks: auth runs, prisma runs, integrations
+run. Tests assert at the HTTP boundary.
+"""
+
+import os
+import tempfile
+from typing import AsyncIterator
+
+import httpx
+import pytest_asyncio
+import yaml
+
+
+MASTER_KEY = "sk-1234"
+
+
+def _write_minimal_proxy_config() -> str:
+    config = {
+        "general_settings": {"master_key": MASTER_KEY},
+        "litellm_settings": {},
+    }
+    database_url = os.environ.get("DATABASE_URL")
+    if database_url:
+        config["general_settings"]["database_url"] = database_url
+
+    f = tempfile.NamedTemporaryFile(mode="w", suffix=".yaml", delete=False)
+    yaml.dump(config, f)
+    f.close()
+    return f.name
+
+
+@pytest_asyncio.fixture(scope="session")
+async def proxy_app():
+    from litellm.proxy.proxy_server import (
+        app,
+        cleanup_router_config_variables,
+        initialize,
+    )
+
+    cleanup_router_config_variables()
+    config_path = _write_minimal_proxy_config()
+    await initialize(config=config_path)
+    yield app
+
+
+@pytest_asyncio.fixture(scope="session")
+async def proxy_client(proxy_app) -> AsyncIterator[httpx.AsyncClient]:
+    transport = httpx.ASGITransport(app=proxy_app)
+    async with httpx.AsyncClient(
+        transport=transport, base_url="http://testserver"
+    ) as client:
+        yield client

tests/proxy_behavior/management/test_smoke.py

@@ -0,0 +1,6 @@
+"""Smoke tests proving the harness boots and talks to the proxy app."""
+
+
+async def test_liveliness(proxy_client):
+    resp = await proxy_client.get("/health/liveliness")
+    assert resp.status_code == 200