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.

src/polygateway/transports/openai_compat.py

@@ -67,7 +67,9 @@ async def _iter_sse_deltas(
         try:
             chunk = json.loads(data)
         except json.JSONDecodeError as exc:
-            raise TransientError(f"SSE 帧畸形(malformed_json): {data[:80]!r}", operation="chat") from exc
+            raise TransientError(
+                f"SSE 帧畸形(malformed_json): {data[:80]!r}", operation="chat"
+            ) from exc
         delta = _sse_delta(chunk, usage_sink)
         if delta is not None:
             yield delta
@@ -95,12 +97,16 @@ def _translate_429(source: SourceConfig, body_text: str, headers: Mapping[str, s
     if err_type == "insufficient_quota":
         return SourceDeadError(
             f"{source.name} 配额耗尽(insufficient_quota)",
-            source_name=source.name, status_code=429, operation="chat",
+            source_name=source.name,
+            status_code=429,
+            operation="chat",
         )
     return TransientError(
         f"{source.name} 限速: 429",
         retry_after_s=_parse_retry_after(headers.get("retry-after")),
-        source_name=source.name, status_code=429, operation="chat",
+        source_name=source.name,
+        status_code=429,
+        operation="chat",
     )
 
 
@@ -164,8 +170,13 @@ class OpenAICompatTransport:
         return client
 
     def _build_payload(
-        self, *, messages: list[dict[str, Any]], source: SourceConfig,
-        profile: ProviderProfile, stream: bool, overlay: dict[str, Any],
+        self,
+        *,
+        messages: list[dict[str, Any]],
+        source: SourceConfig,
+        profile: ProviderProfile,
+        stream: bool,
+        overlay: dict[str, Any],
     ) -> dict[str, Any]:
         payload: dict[str, Any] = {"model": source.model, "messages": messages, "stream": stream}
         if stream:
@@ -178,8 +189,13 @@ class OpenAICompatTransport:
         return payload
 
     async def complete(
-        self, *, messages: list[dict[str, Any]], source: SourceConfig,
-        stream: bool, overlay: dict[str, Any], call_id: str,
+        self,
+        *,
+        messages: list[dict[str, Any]],
+        source: SourceConfig,
+        stream: bool,
+        overlay: dict[str, Any],
+        call_id: str,
     ) -> TransportResult:
         """一次原始调用;HTTP/线路/流式异常按 ARCH §6.2 翻译为领域错误。"""
         profile = get_provider(source.provider, registry=self._registry)
@@ -202,8 +218,12 @@ class OpenAICompatTransport:
             raise TransientError(f"{source.name} 网络错误: {exc}", **ctx) from exc
 
     async def _complete_stream(
-        self, client: httpx.AsyncClient, url: str, payload: dict[str, Any],
-        source: SourceConfig, profile: ProviderProfile,
+        self,
+        client: httpx.AsyncClient,
+        url: str,
+        payload: dict[str, Any],
+        source: SourceConfig,
+        profile: ProviderProfile,
     ) -> TransportResult:
         started = time.monotonic()
         async with client.stream("POST", url, json=payload) as resp:
@@ -236,15 +256,22 @@ class OpenAICompatTransport:
         if salvaged:
             usage_source = "estimated"  # 打捞路径强制 estimated(设计 §6)
         return TransportResult(
-            content=content, thinking=thinking, prompt_tokens=prompt,
-            completion_tokens=completion, usage_source=usage_source,
-            ttft_ms=ttft_ms, max_inter_token_ms=(max_gap if ttft_ms is not None else None),
+            content=content,
+            thinking=thinking,
+            prompt_tokens=prompt,
+            completion_tokens=completion,
+            usage_source=usage_source,
+            ttft_ms=ttft_ms,
+            max_inter_token_ms=(max_gap if ttft_ms is not None else None),
             raw={"usage": sink.get("usage")},
         )
 
     def _check_done(
-        self, sink: dict[str, Any], content_parts: list[str],
-        thinking_parts: list[str], source: SourceConfig,
+        self,
+        sink: dict[str, Any],
+        content_parts: list[str],
+        thinking_parts: list[str],
+        source: SourceConfig,
     ) -> bool:
         """缺 [DONE] 语义(设计 §6): 零内容恒 retry;有内容按 missing_done 策略。"""
         if sink.get("done"):
@@ -268,8 +295,12 @@ class OpenAICompatTransport:
         return content, thinking
 
     async def _complete_once(
-        self, client: httpx.AsyncClient, url: str, payload: dict[str, Any],
-        source: SourceConfig, profile: ProviderProfile,
+        self,
+        client: httpx.AsyncClient,
+        url: str,
+        payload: dict[str, Any],
+        source: SourceConfig,
+        profile: ProviderProfile,
     ) -> TransportResult:
         """非流式快路径(三项目均无,库新增): 单 JSON 响应,仅 total 超时。"""
         resp = await client.post(url, json=payload)
@@ -292,9 +323,14 @@ class OpenAICompatTransport:
         )
         prompt, completion, usage_source = _resolve_usage(body.get("usage") or {}, source)
         return TransportResult(
-            content=content, thinking=thinking, prompt_tokens=prompt,
-            completion_tokens=completion, usage_source=usage_source,
-            ttft_ms=None, max_inter_token_ms=None, raw={"usage": body.get("usage")},
+            content=content,
+            thinking=thinking,
+            prompt_tokens=prompt,
+            completion_tokens=completion,
+            usage_source=usage_source,
+            ttft_ms=None,
+            max_inter_token_ms=None,
+            raw={"usage": body.get("usage")},
         )
 
     async def aclose(self) -> None: