mcp: typed custom methods and extension registration

[sambhav]

Custom method consumers currently repeat the method name and generic types at registration and call sites. This follow-up to #956 packages those details once for extension authors.

As agreed in the review discussion, this PR now covers client-to-server custom methods only. The server-initiated APIs and their additional dispatch maps have been removed.

var Translate = mcp.NewCustomMethod[*TranslateParams, *TranslateResult]("latin/translate")

if err := Translate.RegisterServer(server, handler); err != nil {
    return err
}
if err := Translate.RegisterClient(client); err != nil {
    return err
}
result, err := Translate.Call(ctx, session, &TranslateParams{Text: "hello"})

• CustomMethod wraps the existing registration and call functions with RegisterServer, RegisterClient, and Call.
• RegisterExtension supports package-level setup; ServerOptions.Extensions and ClientOptions.Extensions support per-instance setup. Global extensions run first, in registration order; later registrations of a custom method replace earlier ones. Existing instances are unaffected.
• One helper applies extensions outside the registry lock. Either callback may be nil; callback errors panic during construction.
• The Latin example demonstrates an extension package exposing a simple Translate helper.
• Current main is merged and conflicts resolved.

Validation on Go 1.26.1:

• Custom-method and extension tests pass with -race, including all supported protocol versions, typed-nil params, registration errors, handler errors, ordering, overrides, scope, and concurrent registration.
• Existing multi-round-trip and server-initiated-request rejection tests pass with -race.
• go build ./..., go vet ./..., formatting checks, and the Latin example pass.
• The full local race suite passes with a temporary Go test overlay making the two existing localhost-protection fixtures target 127.0.0.1 instead of their wildcard listener address. This avoids the environment's proxy while preserving their Host headers and assertions; repository test files are unchanged.
• GitHub CI passes the unmodified full suite on Go 1.25 and 1.26, the full race suite, formatting, vet, and staticcheck. Conformance and CodeQL also pass.

[piyushbag]

Looked through the extension registry and bidirectional custom-method wiring. The CustomMethod wrapper and latinext example make the #956 follow-up easy to follow.

Local check:

go test ./mcp/... -count=1

All green on my machine.

One non-blocking suggestion: TestCustomMethods only exercises client→server today. A small round-trip test for AddServerSendingCustomMethod + AddClientReceivingCustomMethod (or ServerCallCustomMethod) would lock in the new path.

[guglielmo-san]

@sambhav thank you for your contribution.
The new mcp protocol disallows Server initiated requests, and should instead be based on the MultiRoundTripRequest
That's why I would lean for not approving this PR

[sambhav]

@sambhav thank you for your contribution. The new mcp protocol disallows Server initiated requests, and should instead be based on the MultiRoundTripRequest That's why I would lean for not approving this PR

gotcha, let me remove those bits. I think the rest of the syntactic sugar still helps though.