chat/completions: streamed logprobs are dropped during reassembly — only last chunk survives

Summary

When a client requests logprobs: true with stream: true, the non-streaming result (and the SSE stream) only contains the logprobs of the last output token. All intermediate token logprobs are lost.

Repro

Request (official DeepSeek API as upstream):

POST /v1/chat/completions
{ "model": "deepseek-v4-flash", "messages": [{ "role": "user", "content": "Reply with exactly: alpha beta gamma delta" }], "thinking": {"type":"disabled"}, "logprobs": true, "top_logprobs": 3, "max_tokens": 100, "stream": true }

Direct to upstream: 6 chunks, 4 content tokens, all 4 carry logprobs ✅
Through Floway: 3 chunks, only the final chunk carries logprobs (1-2 tokens) ❌

Same with non-streaming: Floway returns logprobs.content with 1 token instead of 4.

Root cause

packages/protocols/src/chat-completions/reassemble.ts:

• KNOWN_DELTA_KEYS does not include logprobs → it falls through to captureExtras
• captureExtras uses last-write-wins for non-string non-array values → each chunk overwrites the previous logprobs, leaving only the final chunk's

Same pattern exists in packages/protocols/src/completions/reassemble.ts (accumulator.logprobs = choice.logprobs overwrite).

Expected

Logprobs accumulate across chunks (append to the content array), so logprobs.content has one entry per output token, matching the non-streaming response. OpenAI's streaming wire format sends logprobs per-token in each chunk's delta.logprobs.content — they should be concatenated, not replaced.

Environment

Floway via pnpm workspace monorepo (gateway + protocols packages), upstream = DeepSeek official API (api.deepseek.com, chatCompletions endpoint).

Start in packages/protocols/src/chat-completions/reassemble.ts, tracing KNOWN_DELTA_KEYS and captureExtras, then compare the overwrite in packages/protocols/src/completions/reassemble.ts. Use the supplied streaming and non-streaming DeepSeek reproductions to verify that logprobs.content accumulates one entry per output token rather than retaining only the final chunk.