mcp: lock down Params and Result

Add unexported methods to the Params and Result interface, so that
they're harder to implement outside the mcp package.

It looks like these are the only two interfaces we need to lock down:
others are either intentionally open (Transport, Connection), or already
closed (Session).

Fixes #263

mcp/shared.go

@@ -335,6 +335,9 @@ func setProgressToken(p Params, pt any) {
 
 // Params is a parameter (input) type for an MCP call or notification.
 type Params interface {
+	// mcpParams discourages implementation of Params outside of this package.
+	mcpParams()
+
 	// GetMeta returns metadata from a value.
 	GetMeta() map[string]any
 	// SetMeta sets the metadata on a value.
@@ -356,6 +359,9 @@ type RequestParams interface {
 
 // Result is a result of an MCP call.
 type Result interface {
+	// mcpResult discourages implementation of Result outside of this package.
+	mcpResult()
+
 	// GetMeta returns metadata from a value.
 	GetMeta() map[string]any
 	// SetMeta sets the metadata on a value.
@@ -366,6 +372,7 @@ type Result interface {
 // Those methods cannot return nil, because jsonrpc2 cannot handle nils.
 type emptyResult struct{}
 
+func (*emptyResult) mcpResult()              {}
 func (*emptyResult) GetMeta() map[string]any { panic("should never be called") }
 func (*emptyResult) SetMeta(map[string]any)  { panic("should never be called") }