R6: MCP revision 2026-07-28 on Restlet transports #1640

Follow-up to #1558, whose R1/R2 scope is delivered (#1562, #1627). ext.mcp serves the SDK's stateless 2025-11-25 profile over HTTP and stdio.

R6 targets MCP revision 2026-07-28, the revision Ikanos speaks today, on the MCP Java SDK's modern stateless server (io.modelcontextprotocol.modern, mcp-core 2.2.0-M2, experimental while it is a milestone). The stateless 2025-11-25 server stays for legacy clients. No sessions: the revision drops them, with initialize, GET streams and resumption.

Gate 0 passed in #1662: the modern server behind a test-only Restlet transport mirroring the SDK's servlet transport. server/discover, tools/list, tools/call (plain and base64 Mcp-Name), a streaming reply over core SSE, header/version/method errors and legacy traffic all behave as the revision requires.

Delivery:

1. A Restlet transport for the modern server over HTTP and stdio, within the R1 limits; streaming replies and subscriptions/listen over core SSE; cancellation when the client closes a stream.
2. Both eras on one endpoint: 2026-07-28 _meta to the modern server, initialize and legacy traffic to the stateless 2025-11-25 server.
3. Registration of modern tools, resources, prompts and completions through the SDK's feature repositories.
4. A 2026-07-28 client transport adapted from Ikanos' McpStreamableHttpTransport, until the Java SDK has a modern client.
5. Conformance tests taken from Ikanos' wire tests, corrected where they differ from the revision; interop with Ikanos.

Ikanos side: its client must send client capabilities in _meta, and notifications must get 202, before it interoperates with SDK-based peers (tracked in Ikanos).

Out of scope: sessions; the Tasks extension until the SDK supports it; elicitation and sampling beyond the revision's input_required flow.

Superseded: the first R6 plan targeted the 2025-11-25 Streamable HTTP profile with sessions (listening GET streams, replay, stdio push). Its gate 0 passed in #1659, which stays as evidence should sessions come back.

Related: gRPC R6 (#1561), Smithy R6 (#1559).