fix(tonic-web): don't drop an already-parsed trailer when it arrives alongside data in one poll

[a5x10]

Bug

GrpcWebCall::poll_frame's client-side decode branch (tonic-web/src/call.rs) correctly parses and stashes a trailer frame into self.trailers when it arrives in the same poll_frame call as preceding data — e.g. a fully-buffered response, or any transport that happens to deliver a small response in one read.

But the immediately-following poll_frame call (required by the http_body::Body contract before the caller treats the stream as ended) finds the accumulation buffer empty, takes the FindTrailers::Done(0) arm, and returns None unconditionally, without ever checking self.trailers:

FindTrailers::Done(len) => Poll::Ready(match len {
    0 => None,                                                    // <-- drops an already-parsed trailer
    _ => Some(Ok(Frame::data(buf.split_to(len).freeze()))),
}),

The already-parsed trailer is silently dropped. A tonic client sees:

missing grpc-status trailer, stream was terminated without a final status (possible truncation by a proxy or load balancer)

even though the server sent a perfectly well-formed grpc-web response.

Compare the FindTrailers::Trailer(len) arm a few lines above, which does correctly fall back to checking self.trailers when its own leading data is empty — Done(0) has no equivalent fallback. That asymmetry is the actual bug, not a buffering/timing edge case.

Repro

Found via a real production failure (a tonic-based CLI's grpc-web client against a real deployment). Root-caused offline: captured the actual bytes of a real 232-byte production response, confirmed by hand that they're a perfectly well-formed grpc-web reply (one data frame immediately followed by a complete trailer frame), then reproduced the drop with a minimal, network-free test — included in this PR as call::tests::trailer_not_dropped_when_buffered_with_data. The new test fails without this fix and passes with it (verified both ways before opening this PR).

Fix

One arm, mirroring the FindTrailers::Trailer(len) arm's own existing fallback: check self.trailers.take() before returning None.

Full tonic-web test suite (22 passed, 1 pre-existing #[ignore]d unrelated test, 0 failed) and cargo fmt --check both clean on this branch.

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