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/unit/test_backpressure.py

@@ -210,7 +210,9 @@ class ClockAdvancingTransport:
         self.clock = clock
         self.calls = []
 
-    async def complete(self, *, messages, source, stream, overlay, call_id, reasoning_effort):
+    async def complete(
+        self, *, messages, source, stream, overlay, call_id, reasoning_effort, first_token_event
+    ):
         self.calls.append((source.name, call_id))
         advance, action = self.script.pop(0)
         self.clock.advance(advance)