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_openai_compat.py

@@ -22,10 +22,14 @@ from polygateway.types import SourceConfig
 
 
 def _source(**overrides):
-    base = dict(
-        name="qwen_1", provider="qwen", base_url="https://gw.example/v1",
-        api_key="sk-test", model="qwen-max", timeout_s=5.0,
-    )
+    base = {
+        "name": "qwen_1",
+        "provider": "qwen",
+        "base_url": "https://gw.example/v1",
+        "api_key": "sk-test",
+        "model": "qwen-max",
+        "timeout_s": 5.0,
+    }
     base.update(overrides)
     return SourceConfig(**base)
 
@@ -47,9 +51,7 @@ _USAGE = {"prompt_tokens": 11, "completion_tokens": 7}
 
 def _sse_stream(*frames, done=True):
     text = "".join(frames) + ("data: [DONE]\n\n" if done else "")
-    return httpx.Response(
-        200, content=text.encode(), headers={"content-type": "text/event-stream"}
-    )
+    return httpx.Response(200, content=text.encode(), headers={"content-type": "text/event-stream"})
 
 
 def _transport_for(handler):
@@ -61,8 +63,11 @@ def _transport_for(handler):
 
 async def _complete(transport, source, *, stream=True, overlay=None):
     return await transport.complete(
-        messages=[{"role": "user", "content": "hi"}], source=source,
-        stream=stream, overlay=overlay or {}, call_id="cid-1",
+        messages=[{"role": "user", "content": "hi"}],
+        source=source,
+        stream=stream,
+        overlay=overlay or {},
+        call_id="cid-1",
     )
 
 
@@ -76,8 +81,13 @@ class TestSsePureFunctions:
 
     async def test_iter_deltas_yields_and_flags_done(self):
         async def lines():
-            for raw in [_chunk(content="he"), ": ping", _chunk(reasoning="think"),
-                        _chunk(usage=_USAGE), "data: [DONE]"]:
+            for raw in [
+                _chunk(content="he"),
+                ": ping",
+                _chunk(reasoning="think"),
+                _chunk(usage=_USAGE),
+                "data: [DONE]",
+            ]:
                 for line in raw.splitlines():
                     yield line
 
@@ -97,8 +107,12 @@ class TestSsePureFunctions:
 class TestStreamHappyPath:
     async def test_full_stream_with_usage(self):
         def handler(request):
-            return _sse_stream(_chunk(reasoning="ponder"), _chunk(content="hello"),
-                               _chunk(content=" world"), _chunk(usage=_USAGE))
+            return _sse_stream(
+                _chunk(reasoning="ponder"),
+                _chunk(content="hello"),
+                _chunk(content=" world"),
+                _chunk(usage=_USAGE),
+            )
 
         result = await _complete(_transport_for(handler), _source())
         assert result.content == "hello world"
@@ -152,10 +166,13 @@ class TestNonStreamFastPath:
         def handler(request):
             body = json.loads(request.content)
             assert body.get("stream") is False and "stream_options" not in body
-            return httpx.Response(200, json={
-                "choices": [{"message": {"content": "42", "reasoning_content": "count"}}],
-                "usage": _USAGE,
-            })
+            return httpx.Response(
+                200,
+                json={
+                    "choices": [{"message": {"content": "42", "reasoning_content": "count"}}],
+                    "usage": _USAGE,
+                },
+            )
 
         result = await _complete(_transport_for(handler), _source(), stream=False)
         assert result.content == "42" and result.thinking == "count"
@@ -189,7 +206,8 @@ class TestRequestShaping:
             return _sse_stream(_chunk(content="x"), _chunk(usage=_USAGE))
 
         await _complete(
-            _transport_for(handler), _source(),
+            _transport_for(handler),
+            _source(),
             overlay={"response_format": {"type": "json_object"}},
         )
         assert seen["response_format"] == {"type": "json_object"}
@@ -222,8 +240,9 @@ class TestErrorTranslation:
 
     async def test_retry_after_http_date_ignored(self):
         def handler(request):
-            return httpx.Response(429, content=b"{}",
-                                  headers={"retry-after": "Wed, 21 Oct 2026 07:28:00 GMT"})
+            return httpx.Response(
+                429, content=b"{}", headers={"retry-after": "Wed, 21 Oct 2026 07:28:00 GMT"}
+            )
 
         with pytest.raises(TransientError) as ei:
             await _complete(_transport_for(handler), _source())