add 2026-07-28 MCP spec-compliant v2 server sidecar

[branben]

I wanted to make a discussion around expanding the MCP surface for Omniroute. This is because there was recently a new spec and I've been spending some time contributing to it as it evolves. The most important thing to note is that it's now stateless so the initial handshake probably isn't necessary anymore

PR #12835 adds a v2 MCP server sidecar that implements the MCP 2026-07-28 specification — the largest protocol revision since MCP launched.

The v2 sidecar runs in parallel with the existing v1 server. v1 is untouched. Existing clients keep working. New clients can probe with server/discover and get spec-compliant responses.

Why

The 2026-07-28 spec deprecates sessions and the initialize handshake. Every request becomes self-contained — no session store, no sticky routing. New capabilities land: server/discover, resultType, _meta extraction, header routing (Mcp-Method, Mcp-Name), outputSchema on tools, and MRTR (Multi Round-Trip Requests).

The ecosystem is migrating now. Claude Desktop, Cursor, and Codex connectors are shipping v2 support. Without a v2 endpoint, OmniRoute becomes invisible to the next wave of MCP clients.

What ships in this PR

• server/discover RPC replaces the implicit initialize handshake
• resultType: "complete" on all tool results
• Cacheable list responses (ttlMs + cacheScope)
• _meta extraction (protocolVersion, clientInfo, clientCapabilities)
• Header-based routing
• outputSchema on 8 tool definitions
• MRTR contract support
• 40 tests, all passing

What this PR does NOT do

• Does not replace or deprecate v1
• Does not migrate all 87 v1 tools to v2 (8 to start)
• Does not change any existing client behavior
• Does not implement Tasks or MCP Apps extensions (opt-in under the new Extensions framework)

Open questions

1. Scope: 8 tools in v2 to start. Which tools matter most for the v2 surface?
2. Release strategy: Ship as experimental in v3.8.51, or behind a feature flag (OMNIROUTE_MCP_V2=true)?
3. Migration path: When (if ever) should v1 be deprecated?

Links

• PR #12835
• MCP 2026-07-28 Specification
• Cloudflare MCP v2 Blog

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Looking for feedback on scope, release strategy, and which tools to prioritize for the v2 surface.