feat: add a required first-token event to the transport port

Transport.complete gains the keyword-only first_token_event (no default,
per the port convention): streaming sets it on the first delta, the
non-streaming path accepts it but never sets it, None means the caller
does not observe the first token. All fake/wrapping transports and the
three direct call sites follow the signature; the e2e wrapper forwards.

Red-green evidence: tests/outputs/137/t1/ (batch A TypeError red, then
147 file tests + 1550 unit tests green).

tests/e2e/conftest.py

@@ -1,5 +1,6 @@
 """测试侧独立 HTTP 取证装配；无环境自读取或成功 SSE 预读。"""
 
+import asyncio
 from collections.abc import AsyncIterator, Iterator, Mapping
 from contextlib import AsyncExitStack, asynccontextmanager, contextmanager
 from contextvars import ContextVar
@@ -290,6 +291,7 @@ class ObservedTransport:
         overlay: dict[str, Any],
         call_id: str,
         reasoning_effort: Effort | None,
+        first_token_event: asyncio.Event | None,
     ) -> TransportResult:
         """与生产端口逐参数同签名。"""
         with self._capture.attempt_context(call_id):
@@ -301,6 +303,7 @@ class ObservedTransport:
                 overlay=overlay,
                 call_id=call_id,
                 reasoning_effort=reasoning_effort,
+                first_token_event=first_token_event,
             )
 
     async def embed(