responses: fold EasyInputMessage items that omit type

[ranxianglei]

Problem

OpenAI's Responses API accepts input message items without an explicit type (the EasyInputMessage shorthand: { role: "user", content: "..." }). responsesToCore dispatched its item switch on item.type, so shorthand items fell through the default branch into the opaque preamble and were never folded:

• Hosts sending the shorthand for every turn (the omp adapter's buildResponsesInput) had their ENTIRE conversation bypass compression — fold size stayed 0 and compress always rejected with "0 chars".
• The billion-context proxy serving bare /v1/responses clients (curl etc., all legal shorthand) hit the same hole, surfaced in release v0.1.4 billion-context-omp#12 and the comparison against billion-context.

The omp adapter had to pre-normalize payloads itself (normalizeResponsesPayload); the proxy does not, so the fix belongs in the kernel.

Fix

Normalize type-less items carrying a message role (user/assistant/system/developer) to the canonical typed form before dispatch:

• user/assistant fold as text messages (string or content-part arrays both handled downstream).
• system/developer join systemParts exactly like their typed counterparts (hosts re-inject via injectResponsesDeveloperMessage).
• patchResponsesInput splices folded text back into the slot; re-emitted items carry type: "message" — the shorthand and typed forms are interchangeable on the wire (verified live against SGLang's /v1/responses over a 9-turn tool-loop session).
• Items with neither type nor a message role keep the opaque preamble path (unchanged).

The caller's body object is never mutated — normalization works on copies.

Tests

4 new cases in tests/wire-bili-message-roundtrip.test.ts:

1. Shorthand user/assistant items fold like typed messages (preamble stays empty).
2. patchResponsesInput round-trip: patched text lands in the slot, untouched slots keep content, output in typed form.
3. Shorthand system role joins systemParts alongside instructions.
4. Type-less non-message items stay opaque.

Full pre-flight: tsc --noEmit clean, 395 tests pass, build OK.

Downstream

• billion-context-omp: the adapter-local normalizeResponsesPayload becomes redundant once it pins a kernel release with this fix (kept for now; harmless double-normalize).
• billion-context (proxy): shorthand bodies served to bare clients become compressible with no proxy-side change.

Refs ranxianglei/billion-context-omp#12