[backlog] Multi-turn /v1/responses reasoning content (upstream #538)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #538
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: partial · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    Responses surface: `handler/proxy/router.go` `/responses`, transform reasoning parsers `transform/shared/chatFormatsCore.go` `parseResponsesReasoning` / `ResponsesReasoningByIndex`; compact sanitize `transform/openai/responses/compact.go`. No dedicated fix ensuring multi-turn reasoning items always carry required `content` for Hermes/Codex second turn.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Multi-turn Hermes/Codex `/v1/responses` second turn accepts prior reasoning items with required content

• Compact/sanitize path does not drop required reasoning content

• Fixture covers reasoning item round-trip across turns

• Error message when client omits required fields is explicit

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.