[Bugfix][Rust Frontend] Handle zero-column logprobs payloads without panicking

[FeathBow]

Purpose

The engine-core protocol can carry a logprobs payload with zero scored positions: the natural encoding of an empty position list is a pair of [0, 0] tensors, which is exactly what the crate's own WireLogprobs::from_direct produces for Logprobs { positions: [] }. The decoder passes the column count straight into slice::chunks, which panics unconditionally on zero (chunk size must be non-zero), so such a payload panics during decoding. The stock Python runners never send this shape (Model Runner V1 emits [0, k+1], Model Runner V2 omits the payload entirely), but the engine-core protocol is a public boundary for compatible engine cores, and a panic there is the wrong failure mode.

This PR handles both zero-column cases explicitly: a zero-row payload decodes to empty Logprobs, while a zero-column payload that still claims rows is rejected with a structured ExtValueDecode error. Row-content validation is unchanged.

Not a duplicate: no open PR touches this decode path. It is independent of the single-token prompt-logprobs route fix (separate layer; neither requires the other).

No model evaluation is needed: this is protocol decoding only; no model output is involved, and the fix is covered by protocol-level unit tests. AI assistance was used for this change; the submitter reviewed every changed line and ran the tests below.

Test Plan

cd rust
cargo nextest run -p vllm-engine-core-client
cargo clippy -p vllm-engine-core-client --all-targets -- -D warnings

New tests: decodes_zero_row_logprobs_as_empty (shapes [0, 0] and [0, 3]) and rejects_zero_column_logprobs_with_rows (shape [2, 0] must yield a decode error, not a panic). cargo nextest run -p vllm-server as a downstream ripple check.

Test Result

Without the fix, both new tests fail by panic:

thread 'protocol::logprobs::tests::decodes_zero_row_logprobs_as_empty' panicked at src/engine-core-client/src/protocol/logprobs.rs:275:14:
chunk size must be non-zero

With the fix: 95 tests run, 95 passed; clippy clean. vllm-server downstream: 305 tests run, 304 passed (the one failure is a pre-existing TLS handshake-timing test, identical on the unmodified base).

[claude]

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[BugenZhao]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: 23fa5b110c

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[BugenZhao]

The stock Python runners never send this shape (Model Runner V1 emits [0, k+1], Model Runner V2 omits the payload entirely), but the engine-core protocol is a public boundary for compatible engine cores, and a panic there is the wrong failure mode.

IIUC the context is openinfer-project/openinfer#721?

[BugenZhao]

LGTM

[FeathBow]

The stock Python runners never send this shape (Model Runner V1 emits [0, k+1], Model Runner V2 omits the payload entirely), but the engine-core protocol is a public boundary for compatible engine cores, and a panic there is the wrong failure mode.

IIUC the context is openinfer-project/openinfer#721?

Not openinfer#721—it surfaced while reviewing openinfer#722, the fix for openinfer#720. This PR is independent protocol hardening.