test: unit suite for the gateway invariants plus an integration slice

Covers routing, failover, the circuit breaker, structured-output retries, the
cache-prefix validator, cost computation including cached-token pricing, the
daily budget breaker, the quota protocol, the task state machine and log
redaction. HTTP is mocked -- no unit test spends money. The integration test
runs the real slice against PostgreSQL and Redis with a faked provider.

tests/conftest.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import os
+from collections.abc import Iterator
+from pathlib import Path
+
+import pytest
+
+# Unit tests never touch a real database, Redis or LLM provider. Setting these
+# before any import keeps `packages.shared.config` deterministic.
+os.environ.setdefault("APP_ENV", "local")
+os.environ.setdefault("JWT_SECRET", "unit-test-secret-value-not-used-in-any-real-env")
+os.environ.setdefault("DEEPSEEK_API_KEY", "unit-test-placeholder")
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+@pytest.fixture
+def repo_root() -> Path:
+    return REPO_ROOT
+
+
+@pytest.fixture
+def prompts_dir(tmp_path: Path) -> Iterator[Path]:
+    """A minimal on-disk prompt tree with a clean, cacheable system prefix."""
+    root = tmp_path / "prompts"
+    (root / "agent_chat").mkdir(parents=True)
+    (root / "agent_chat" / "v1.txt").write_text(
+        "[system]\n你是穿搭顾问助手。只回答穿搭相关问题。\n\n[user]\n{user_input}\n",
+        encoding="utf-8",
+    )
+    (root / "agent_chat" / "v2.txt").write_text(
+        "[system]\n你是穿搭顾问助手。回答要具体可执行。\n\n[user]\n{user_input}\n",
+        encoding="utf-8",
+    )
+    yield root