feat: let chat() take a tenant and caller-defined dimensions

Validation runs before the request enters the onion: every failure inside it
is downgraded to a warning by the telemetry layer, so validating in there
would not validate anything.

The dimensions stay out of the cache key — cache_namespace already carries
tenant isolation, and folding meta in would cold-start every existing entry.

src/polygateway/client.py

@@ -34,7 +34,12 @@ from polygateway.sources import (
     SourceCooldownMemo,
 )
 from polygateway.transports.openai_compat import OpenAICompatTransport
-from polygateway.types import ChatRequest, LLMResponse, validate_request_overlay
+from polygateway.types import (
+    ChatRequest,
+    LLMResponse,
+    validate_caller_dimensions,
+    validate_request_overlay,
+)
 
 if TYPE_CHECKING:
     from collections.abc import Awaitable, Iterable, Mapping
@@ -205,12 +210,18 @@ class GatewayClient:
         structured: type[BaseModel] | Literal["json"] | None = None,
         stream: bool = True,
         overlay: Mapping[str, Any] | None = None,
+        tenant_id: str | None = None,
+        meta: Mapping[str, Any] | None = None,
     ) -> LLMResponse:
         """一次治理调用(签名冻结,ARCH §5.2;与三项目 LLMProvider 协议兼容)。
 
         `overlay` 是采样参数覆盖层(`temperature`/`seed`/`max_tokens` 等),优先级
         高于源级 `extra_body`、低于结构化输出的注入。带默认值的 keyword-only
         参数不影响既有调用点(issue #4)。
+
+        `tenant_id` 与 `meta` 是调用方自定义维度,只进遥测、**不进缓存 key**
+        (租户隔离由 `cache_namespace` 负责,ARCH §7.5);前者享有真实列待遇
+        (可挂 RLS、可进复合索引),后者是任意 KV 容器(issue #11)。
         """
         if structured is not None and not self._structured_available:
             raise ImportError(
@@ -221,6 +232,12 @@ class GatewayClient:
         # 造成的竞态。同一份快照填 overlay 与 sampling——前者会被结构化注入,
         # 后者跨层恒定,供缓存 key 与遥测读取(设计决策 A/B/E)
         sampling = validate_request_overlay(overlay or {}, origin="chat(overlay=...)")
+        # 同理必须在洋葱之外: 洋葱内的一切失败都被遥测层降级成 warning(库铁律
+        # 「遥测写失败降级不冒泡」),校验放里面等于没有校验——非法维度会变成
+        # 静默丢失的遥测行,而调用照常发出(issue #11 §4.2)
+        dimension_tenant_id, dimensions = validate_caller_dimensions(
+            tenant_id, meta, origin="chat(tenant_id=..., meta=...)"
+        )
         request = ChatRequest(
             messages=messages,
             session_id=session_id,
@@ -231,6 +248,8 @@ class GatewayClient:
             stream=stream,
             overlay=sampling,
             sampling=sampling,
+            tenant_id=dimension_tenant_id,
+            meta=dimensions,
         )
         return await self._handler(request)