server/discover is not customizable or host-invokable (no DiscoverHandler option / session Discover API)

Use case

We run a multi-tenant MCP gateway (Arcade) on v1.7.0-pre.2, stateless StreamableHTTP, one logical server per gateway. Two things we cannot do today:

1. Customize the server/discover payload beyond what ServerOptions statics allow (e.g. tenant-specific supportedVersions, augmenting the result at build time rather than post-hoc).
2. Invoke discover from the host (for health/introspection: "what would this session advertise right now?") without synthesizing an HTTP request against ourselves.

Current behavior

• server/discover is handled entirely internally: unexported Server.discover (server.go), registered in serverMethodInfos, special-cased in dispatch. It builds DiscoverResult from s.capabilities(), s.impl, s.opts.Instructions, and the session version list.
• No ServerOptions field touches discover (checked the full option set on pre.2), and there is no exported Discover(...) on Server/ServerSession.

Why middleware isn't a workaround

Receiving middleware can type-assert *DiscoverResult and mutate it post-hoc, keyed off the method string — reactive, fragile, and only for inbound requests. It cannot let the host invoke discover, and it runs strictly after the SDK computed the payload.

Per the maintainers' stance in #565 (keep the API small), the concrete need here is not "export the internals" but a supported seam:

Proposal (either would do)

type ServerOptions struct {
    // DiscoverHandler, if non-nil, can augment the server/discover
    // response after the SDK builds it.
    DiscoverHandler func(context.Context, *DiscoverRequest, *DiscoverResult) (*DiscoverResult, error)
}

or an exported, host-callable form:

func (ss *ServerSession) Discover(ctx context.Context) (*DiscoverResult, error)

Related but distinct: #1034 fixed a discover/initialize parity bug; this asks for a customization/introspection seam.

🐕 Written by Kyoto, an AI agent, on Pascal's behalf —

Start by reading server.go, the unexported Server.discover implementation, serverMethodInfos, and the dispatch special case to understand how server/discover is assembled. Compare the ServerOptions and ServerSession APIs, then define and validate one supported customization or host-invocation seam that preserves the existing discover behavior while enabling tenant-specific results and session introspection.