fix: treat response.incomplete as a terminal partial turn

[TheGreatAxios]

Truncated turns are turns, not protocol violations.

Streaming parse no longer throws ProtocolMismatchError on response.incomplete: the envelope is schema-validated, then ignored. Deltas already delivered stay, isStreamTerminal still ends the stream, and no usage is reported. Non-streaming parse decodes an incomplete body's output the same way and withholds its usage. This restores the 0.1.0-era lenient contract that the Codex adapter and downstream hosts depend on (partial text, terminal, no usage); the strict throw from #23 broke backends that truncate routinely on max_output_tokens.

Fixes CL-9472

[TheGreatAxios]

Review · Comment

A response.incomplete stream event or an incomplete JSON body ends the turn with the partial content already decoded, and reports no usage.

Findings

• src/protocol/iterator.ts:481 / src/protocol/iterator.ts:746 — usage is dropped on the incomplete path. The tokens in that envelope are what the vendor actually bills for the truncated turn (input and the output emitted before the cap), so withholding them undercounts any accounting built on inference.usage. Could we emit usage whenever the envelope carries it, the same way response.completed does at line 443, and keep the "partial" concern to the content rather than the counts? If the intent is that truncated usage is untrustworthy for a specific vendor, that reads like a quirk switch rather than a blanket drop.

Notes

• Neither inference.done nor the turn carries a stop reason, so a consumer cannot tell a truncated turn from a complete one. That is a protocol limit, not something this branch can fix, but worth a line in the comment so the next reader knows the loss is intentional.
• bun run check exits 0 (49 unit tests pass); bun test e2e passes 5 with 2 skipped.