docs: refresh the corpus; pin the streaming usage chunk that stopped existing

[lroolle]

DeepSeek changed the documented streaming wire shape on 2026-08-27, in both Chat Completions and the FIM beta, with no changelog entry.

Before — stream_options.include_usage added one more chunk before data: [DONE]:

{"choices":[],"usage":{"prompt_tokens":11,"completion_tokens":2}}

Now — no separate usage chunk is emitted. The statistics ride on the last content chunk:

{"choices":[{"delta":{},"finish_reason":"stop"}],"usage":{"prompt_tokens":11,"completion_tokens":2}}

A parser that recognised the usage chunk by its empty choices now finds usage on no chunk at all and prices the call at zero. Nothing errors. The bill just disappears — the worst failure this tool has, because the number at the bottom is the whole point of it.

What was and wasn't broken

Our assembler already takes usage from whatever chunk carries it (internal/deepseek/chat.go), so nothing was broken. Nothing tested it either — no test in this repo has ever streamed a usage-bearing chunk, so the one line standing between us and a silent zero was unguarded.

TestStreamTakesUsageFromTheLastContentChunk pins the shape the docs describe rather than the shape we happened to be written against.

The test was checked for teeth rather than assumed to have them. Narrowing the assembler to the old rule, written the obvious way:

if chunk.Usage != nil && len(chunk.Choices) == 0 {

--- FAIL: TestStreamTakesUsageFromTheLastContentChunk
    client_test.go:219: no usage assembled: a streamed call that reports no tokens cannot be priced

Corpus

Repacked from the mirror at thevibeworks/deepseek-docs@2e1187d. 75 pages, en minus dsh, as before; create-chat-completion.md and create-completion.md now carry the new wording in docs ask.

make check: All checks passed.