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).
iomgaa