SEP-2322: Multi Round-Trip Requests - #2322

…-2322

## Motivation and Context

Final step of SEP-2322 (modelcontextprotocol/modelcontextprotocol#2322) for the 2026-07-28 MCP spec release.
The client so far only recognized `input_required` results by raising `InputRequiredError`; this adds the resume loop,
mirroring the Python SDK's high-level `Client` driver and the TypeScript SDK's `runInputRequiredDriver`.

- The driver answers embedded requests with the handlers already registered through `on_elicitation`,
  `on_sampling`, and the new `on_roots`. There is no second registration path: a server can ask for input two
  ways, as a real request mid-call and as an entry in an `input_required` result, and one registration covers
  both. This follows the TypeScript driver, which dispatches "to the client's already-registered handlers
  (elicitation, sampling, roots - one generic engine, no per-feature API)".
- `on_elicitation` and `on_sampling` no longer refuse a transport without `on_server_request`. They record
  the handler either way and wire it to the transport only when it can carry server-to-client requests. Refusing
  would have locked stdio clients, and every 2026-07-28 connection, out of the only route the spec leaves:
  the modern lifecycle forbids server-to-client requests, so an embedded request is how it asks.
- `MCP::Client.new` gains `input_required_max_rounds:` (default 10, the TypeScript and Python default).
- Once a handler is registered, `call_tool`, `get_prompt`, and `read_resource` resume automatically:
  each `inputRequests` entry is fulfilled by the matching handler and the ORIGINAL request is re-issued with
  `inputResponses` under the same keys plus the byte-exact echoed `requestState`, on a fresh JSON-RPC id per leg
  (modern envelope stamping happens in the transports per send, so every leg carries the SEP-2575 triple).
  A `requestState`-only result (load shedding) retries after an exponential backoff from 50ms to a 250ms cap,
  matching the Python SDK; every leg counts against the round cap, whose exhaustion raises `InputRequiredError`
  carrying the last result.
- A requested kind without a matching handler falls back to the manual path by raising `InputRequiredError`,
  and clients with no handlers keep the exact pre-existing behavior. For manual driving,
  `call_tool` / `get_prompt` / `read_resource` gain `input_responses:` and `request_state:` keyword arguments
  that ride at the params top level. Non-MRTR methods such as `tools/list` keep raising unconditionally.

The conformance client rides the same change: it derives its lifecycle from the wire version the harness names per run
(`MCP_CONFORMANCE_PROTOCOL_VERSION`), connecting with `mode: :modern` for the 2026 draft and keeping the pinned legacy
handshake through 2025-11-25, retrying a modern connect once when the server rejects the first request with `-32022`
naming its supported versions. New scenario branches drive `sep-2322-client-request-state` through this driver
(an accepting `on_elicitation` handler plus the four `test_mrtr_*` tools), `request-metadata`, both SEP-2243 header
scenarios, and `json-schema-ref-no-deref`. The expected-failures baseline drops the six scenarios this stack resolves,
leaving only the not-scored extensions and the post-anchor `json-schema-2020-12-preservation`.

Verified end-to-end over stdio: a modern connection (`connect(mode: :modern)` with declared form-elicitation capability)
against a server whose tool returns `InputRequiredResult` under `RequestStateSecurity` sealing completes in two legs with
the handler supplying the answer, and a handler-less client receives `InputRequiredError` with the sealed opaque state.

Part of #382.

## How Has This Been Tested?

New tests in `test/mcp/client_test.rb` (Mocha sequenced transports) cover: the two-leg happy path asserting preserved original params,
key-matched `inputResponses`, byte-exact `requestState` echo, and a fresh id per leg; the exponential backoff sequence for
`requestState`-only legs (stubbed `sleep`); round-cap exhaustion raising `InputRequiredError` with the last state; the unhandled-kind
fallback; the manual `input_responses:`/`request_state:` keyword arguments; and the `prompts/get` driver path. The existing SEP-2322
recognition tests pin the no-handler behavior unchanged.

Three tests cover the shared-registration design specifically: the driver resolving an embedded request on a transport that
has no `on_server_request` at all, an `on_roots` handler answering an embedded `roots/list`, and `on_elicitation` and
`on_sampling` accepting a registration on such a transport rather than raising, which replaces the two tests that pinned
the old refusal.

`bundle exec rake` (tests, RuboCop, and conformance baseline) passes, plus the stdio end-to-end script described above.

With the separate SEP-2243 mirroring change applied alongside, every scored scenario of the frozen 2026-07-28 client leg passes
(`sep-2322-client-request-state` 5/5, `request-metadata` 5/5 with no version-retry warning, `http-custom-headers` 18/18,
`http-invalid-tool-headers` 11/11), and the `--requirements` client legs at both revisions report failures only
in not-scored scenarios. The full conformance task passes its baseline check on both legs.

## Breaking Changes

`on_elicitation` and `on_sampling` no longer raise `ArgumentError` on a transport without `on_server_request`; they
register the handler for the `input_required` route instead. Code that relied on the refusal to detect an unsuitable
transport needs another check. Everything else is additive: the new keyword arguments default to `nil`, and with no
handler registered every code path is identical to before.