vmcp/server: expose a hook for embedders to drive MCP elicitation

[JAORMX]

Problem

server.Server builds a composer.ElicitationProtocolHandler internally (pkg/vmcp/server/server.go:347 via composer.NewDefaultElicitationHandler) and feeds it a mark3labs *mcpserver.MCPServer via the unexported newSDKElicitationAdapter (pkg/vmcp/server/sdk_elicitation_adapter.go:39). Three seams are closed to embedders:

1. *Server.mcpServer field is unexported.
2. No MCPServer() accessor or equivalent on *Server.
3. newSDKElicitationAdapter is unexported.

Use case

Embedders who wrap toolhive's composer in their own pipeline (gateway-level audit, custom elicitation surface, ahead-of-time elicitation schema validation) cannot re-target their elicitation flow through the server toolhive constructs internally. Holding a parallel mark3labs server does not work because /mcp requests are served by toolhive's internal instance, so server.ClientSession correlation lands on the wrong server.

Proposed API

Pick one (in order of smallest surface):

a) Add (*Server).MCPServer() *mcpserver.MCPServer accessor. Minimal disclosure, lets embedders install their own elicitation handler via server.WithElicitation(...) against the actual serving server.

b) Export NewSDKElicitationAdapter(srv *mcpserver.MCPServer) composer.ElicitationProtocolHandler. Useful in its own right for embedders that construct their own composer separately from server.New.

c) Add server.WithElicitationHandler(h composer.ElicitationProtocolHandler) Option so callers can inject their own handler end-to-end, bypassing the SDK adapter entirely.

Scope

• Additive.
• Choice (a) is the thinnest change; (b) is orthogonal and useful on its own; (c) is the most complete but requires coordinating with the composer's existing handler-wiring logic.