R6: Modern MCP core, with legacy MCP as a deletable bridge edge #1671

Follow-up to #1640. McpServerAdapter now serves both eras (#1664, #1665), but with the legacy model at the core: requests with MCP-Protocol-Version: 2026-07-28 go to the SDK's modern stateless server, everything else to the 2025-11-25 path, and interceptors and tool filters run on 2025-11-25 types, with modern tools seen through their legacy projection.

This issue inverts that. The modern API becomes the only registration and policy model, and legacy MCP (2025-11-25 and earlier) is accepted only as a translation edge in front of it. The edge exposes nothing that 2026-07-28 deprecates or removes, so it can be deleted when the deprecation window closes.

What the legacy edge must not expose:

Delivery:

1. Modern core. Interceptors and tool filters run on modern types. The 2025-11-25 projection is produced at the edge, after policy, so policy runs once for both eras.
2. Server edge for legacy clients. initialize is answered from the same data as server/discover, advertising only tools, resources and prompts. No server-to-client requests (no sampling, no roots/list, no request-style elicitation). A modern input_required (MRTR) result returns an error to legacy clients; it is not mapped to legacy elicitation.
3. Code layout. The edge lives in org.restlet.ext.mcp.internal.legacy, behind one seam (McpLegacyEdge) that the core calls for non-modern requests. No new Maven module: the SDK ships both servers in the same mcp-core artifact, so a module would add release overhead without removing a dependency. Dropping the bridge later means deleting one package and the seam.
4. Public API, breaking. McpServerAdapter and McpClientAdapter getters and setters that take 2025-11-25 SDK stateless specifications are removed and replaced by modern-only registration. No deprecation step: 3.0 is still at M1, so legacy types leave the public API now.
5. Architecture test (ArchUnit, or a plain import-scanning test to avoid a new test dependency):
   • Nothing outside internal.legacy depends on it except through the seam.
   • Only internal.legacy imports the SDK's legacy server and spec packages; everything else uses io.modelcontextprotocol.modern, public API included, with no allow-list.
   • internal.legacy references no SDK logging, sampling or roots types. The server edge never issues Mcp-Session-Id; the client edge sends one only when the caller opts in through the legacySession* properties (R7: Dual-era MCP client: detect the server's era and fall back to legacy servers #1670).
6. Behavior tests:
   • A legacy initialize response advertises no logging, roots or sampling capability and carries no session ID.
   • Legacy GET returns 405 and logging/setLevel returns -32601.
   • The same registrations serve a legacy and a modern client, with policy applied once.

The client side of the bridge (fallback to legacy servers) is #1670, under the same limits.

Open questions:

• Should tools that can return input_required be hidden from legacy tools/list rather than fail on call?
• Should the legacy initialize error code match Ikanos (-32022) or the SDK (-32602)?
• Which release removes the legacy edge, given the twelve-month deprecation window from 2026-07-28?

Design memo: https://claude.ai/artifact/73QEwzKyg79YHVHjAyHdLd

Related: #1640, #1670, #1664, #1665.