[backlog] previous_response_id parameter handling (upstream #504)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #504
    - 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 compact/sanitize strips some fields (`stream_options`/`store` in `transform/openai/responses/compact.go`); `previous_response_id` handling not clearly complete — clients report unsupported parameter errors.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] `previous_response_id` accepted, stripped, or translated per supported upstream capabilities

• Unsupported upstream paths return clear errors instead of opaque 400s where possible

• Compact/sanitize policy documented for responses fields

• Tests cover presence/absence of previous_response_id

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