Modern dispatch answers ping, which 2026-07-28 removed, and omits the required resultType

[JAORMX]

Summary

Two small conformance problems on the same line of dispatchModern, both pre-existing and currently inert.

1. ping was removed in 2026-07-28. grep -ci ping against schema/draft/schema.ts returns 0 (it returns 7 against schema/2025-11-25/schema.ts, where PingRequest { method: "ping" } is defined). go-sdk agrees — mcp/server.go:1880 lists methodPing among methods "removed in the new protocol" and answers -32601. Per SEP-2575's own rule for an unsupported RPC, -32601 (HTTP 404) is the conformant answer, which means the default: branch of the dispatch switch was already correct and the explicit ping case should not be there.

2. The result omits resultType, which is a MUST. writeModernResult(w, id, struct{}{}) emits a bare {}. schema/draft/schema.ts:229 states "Servers implementing this protocol version MUST include this field" for resultType, on every result. So even if ping were still a valid Modern method, this response would be non-conformant.

A comment at pkg/vmcp/server/modern_dispatch.go:110-111 asserts that a bare {} is "the correct, spec-matching result" — wrong on both counts, and its cited authority is unreachable for a Modern request. That comment has been corrected in #6050; the behaviour was deliberately left alone there to avoid scope creep on a pre-existing path with existing tests.

Impact: currently none

A Modern go-sdk client never pings — startKeepalive sits past an early return at client.go:357. So nothing reaches this path today, which is why it was left rather than fixed mid-review.

It matters if a non-go-sdk Modern client ever pings, or if someone reads this path as a template for another method's result shape. The second is the likelier harm: resultType is required on every Modern result, and a copy of this code would inherit the omission.

Suggested fix

Delete the ping case so it falls to default: and returns -32601, matching both the spec and the reference implementation. Check whether any existing test asserts the current {}/200 behaviour and update it — a test pinning a non-conformant response is worth removing regardless.

While there, audit other writeModernResult call sites for the same resultType omission.

Refs #6050, #5959.