## Why
`pr17088` adds typed server-originated request/response plumbing, but
successful client responses are still erased into bare JSON-RPC `result`
values before app-server can make any typed decision about them.
This precursor PR keeps successful client responses typed until the
outgoing response seam. It is intentionally limited to
protocol/app-server plumbing so the analytics behavior change can review
separately on top.
## What changed
- Add `ClientResponsePayload` as the pre-serialization client response
body type.
- Route app-server successful response paths through the typed payload
seam while preserving existing handler-local analytics behavior.
- Keep `InterruptConversation` JSON-RPC-only because it has no
`ClientResponse` variant.
- Move the new payload conversion tests into a dedicated protocol test
module.
## Verification
- `cargo check -p codex-app-server`
- `cargo test -p codex-app-server-protocol`
rhan-oai