mcp: advertise a capability if and only if added (#93)

If tools are added to a Server when Run is called, it will send
toolCapabilities to the client. Otherwise it won't. Ditto for prompts
and resources.

For #56.

mcp/streamable_test.go

@@ -159,8 +159,6 @@ func TestStreamableServerTransport(t *testing.T) {
 		Capabilities: &serverCapabilities{
 			Completions: &completionCapabilities{},
 			Logging:     &loggingCapabilities{},
-			Prompts:     &promptCapabilities{ListChanged: true},
-			Resources:   &resourceCapabilities{ListChanged: true},
 			Tools:       &toolCapabilities{ListChanged: true},
 		},
 		ProtocolVersion: latestProtocolVersion,