feat(mcp): export initialized result in client session (#320)

Export ServerCapabilities and related subtypes and update protocol,
server, and tests to use exported types. Add InitializeResult method on
ClientSession for accessing the initialize result from sessions.

Fixes: https://github.com/modelcontextprotocol/go-sdk/issues/166

Co-authored-by: Shashank Pachava <spachava753@gmail.com>

mcp/client.go

@@ -177,6 +177,12 @@ type clientSessionState struct {
 	InitializeResult *InitializeResult
 }
 
+func (cs *ClientSession) InitializeResult() *InitializeResult { return cs.state.InitializeResult }
+
+func (cs *ClientSession) setConn(c Connection) {
+	cs.mcpConn = c
+}
+
 func (cs *ClientSession) ID() string {
 	if c, ok := cs.mcpConn.(hasSessionID); ok {
 		return c.SessionID()