### Why
Remote streamable HTTP MCP needs a transport-shaped executor primitive
before the MCP client can move network I/O to the executor. This layer
keeps the executor unaware of MCP and gives later PRs an ordered
streaming surface for response bodies.
### What
- Add typed `http/request` and `http/request/bodyDelta` protocol
payloads.
- Add executor client helpers for buffered and streamed HTTP responses.
- Route body-delta notifications to request-scoped streams with sequence
validation and cleanup when a stream finishes or is dropped.
- Document the new protocol constants, transport structs, public client
methods, body-stream lifecycle, and request-scoped routing helpers.
- Add in-memory JSON-RPC client coverage for streamed HTTP response-body
notifications, with comments spelling out what the test proves and each
setup/exercise/assert phase.
### Stack
1. #18581 protocol
2. #18582 runner
3. #18583 RMCP client
4. #18584 manager wiring and local/remote coverage
### Verification
- `just fmt`
- `cargo check -p codex-exec-server -p codex-rmcp-client --tests`
- `cargo check -p codex-core --test all` compile-only
- `git diff --check`
- Online full CI is running from the `full-ci` branch, including the
remote Rust test job.
Co-authored-by: Codex <noreply@openai.com>
---------
Co-authored-by: Codex <noreply@openai.com>
