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_usage_source_domain.py

@@ -139,6 +139,7 @@ async def test_salvage_override_stays_in_domain(usage):
         overlay={},
         call_id="cid",
         reasoning_effort=None,
+        first_token_event=None,
     )
     assert result.usage_source in USAGE_SOURCES