feat: add gateway client with env-driven assembly

Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.

tests/unit/test_cache.py

@@ -15,12 +15,21 @@ _MSGS = [{"role": "user", "content": "hi"}]
 
 
 def _resp(content="cached", **overrides):
-    base = dict(
-        content=content, thinking="", model="m", provider="p",
-        prompt_tokens=1, completion_tokens=2, latency_ms=30,
-        ttft_ms=5.0, max_inter_token_ms=2.0, cache_hit=False, call_id="orig",
-        source_name="s1", usage_source="measured",
-    )
+    base = {
+        "content": content,
+        "thinking": "",
+        "model": "m",
+        "provider": "p",
+        "prompt_tokens": 1,
+        "completion_tokens": 2,
+        "latency_ms": 30,
+        "ttft_ms": 5.0,
+        "max_inter_token_ms": 2.0,
+        "cache_hit": False,
+        "call_id": "orig",
+        "source_name": "s1",
+        "usage_source": "measured",
+    }
     base.update(overrides)
     return LLMResponse(**base)
 
@@ -46,21 +55,33 @@ class TestKeyFormula:
 
     def test_multimodal_part_digested_not_inlined(self):
         big_b64 = "data:image/png;base64," + "A" * 1_000_000
-        messages = [{"role": "user", "content": [
-            {"type": "image_url", "image_url": {"url": big_b64}},
-            {"type": "text", "text": "describe"},
-        ]}]
+        messages = [
+            {
+                "role": "user",
+                "content": [
+                    {"type": "image_url", "image_url": {"url": big_b64}},
+                    {"type": "text", "text": "describe"},
+                ],
+            }
+        ]
         digested = digest_messages(messages)
         payload = json.dumps(digested, ensure_ascii=False)
         assert len(payload) < 500  # 大图不进 canonical_json
         expected = hashlib.sha256(big_b64.encode()).hexdigest()
         assert expected in payload  # 但字节变化仍改变 key
         # 图像字节变化 → key 变
-        messages2 = [{"role": "user", "content": [
-            {"type": "image_url", "image_url": {"url": big_b64[:-1] + "B"}},
-            {"type": "text", "text": "describe"},
-        ]}]
-        assert build_cache_key("m", messages, "p", None) != build_cache_key("m", messages2, "p", None)
+        messages2 = [
+            {
+                "role": "user",
+                "content": [
+                    {"type": "image_url", "image_url": {"url": big_b64[:-1] + "B"}},
+                    {"type": "text", "text": "describe"},
+                ],
+            }
+        ]
+        assert build_cache_key("m", messages, "p", None) != build_cache_key(
+            "m", messages2, "p", None
+        )
 
 
 class _Terminal:
@@ -76,7 +97,12 @@ class _Terminal:
 
 
 def _mw(backend, **kwargs):
-    defaults = dict(backend=backend, model_fingerprint="m", default_namespace="proj", ttl_s=3600)
+    defaults = {
+        "backend": backend,
+        "model_fingerprint": "m",
+        "default_namespace": "proj",
+        "ttl_s": 3600,
+    }
     defaults.update(kwargs)
     return CacheMW(**defaults)