Carry forward safe Responses replay diagnostics on current SDK #446

Scope

Carry forward the independently useful diagnostics and SDK-level regression coverage from the closed/superseded draft PR #218, rebuilt against the repository's current pinned pi-ai / DSH baseline. This issue is a follow-up request, not evidence that the work is already implemented.

The prior PR added structured, allowlisted reasons for rejected Responses replay shapes (including reasoning without encrypted content, incomplete items, reference-only items, and previous-response references), a locally owned zero-based input index, safe type/status values and field-presence booleans, while retaining the existing message-only INVALID_REQUEST surface. Its tests also covered malicious/changing getters and a synthetic pi-ai Responses stream. The old draft's results relied on pi-ai 0.87.1 and must not be treated as current SDK evidence.

Acceptance criteria

• Inspect the current plugin and SDK APIs, then implement only plugin-owned safe structural diagnostics through the existing native error surface.
• Add focused synthetic SDK/integration regressions against the exact current lockfile dependencies. Verify a rejected request does not dispatch network I/O, does not mutate original history, preserves encrypted bytes and call_id on valid replay, and never discloses unknown or hostile fields.
• Keep current replay, authorization, account isolation, cancellation and transaction semantics intact. Do not admit new replay shapes, capture payloads, retry automatically, delete history, switch models, or change DSH Core.
• Update directly relevant English and Chinese documentation.
• Run focused tests and the repository's documented validation gates; report environmental limits without claiming unrun checks passed.

Context and boundaries

#221 closed #217 but explicitly did not replace the unique diagnostic and SDK reproduction scope in #218. Main has since changed through #227, #281, #319 and #327, so the old branch must not be cherry-picked as-is. Start from current main and preserve the old branch for reference. This work is separate from the concurrent install/CI dependency-guard task; keep that CI/tooling scope untouched.

Do not merge, publish, install, restart, or make real model calls as part of this issue.