internal/mcp: handle synthetic responses in conformance test

Modify the conformance test driver to support outgoing responses.
They are returned for incoming requests in order.

Add a test that returns a file resource, which involves the server
sending a roots/list request to the client.

Change-Id: Ie7eb69a8c99ccbf11721518cfffb21244c03e532
Reviewed-on: https://go-review.googlesource.com/c/tools/+/677475
Commit-Queue: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/jsonrpc2/messages.go

@@ -165,7 +165,7 @@ func DecodeMessage(data []byte) (Message, error) {
 		return nil, fmt.Errorf("unmarshaling jsonrpc message: %w", err)
 	}
 	if msg.VersionTag != wireVersion {
-		return nil, fmt.Errorf("invalid message version tag %s expected %s", msg.VersionTag, wireVersion)
+		return nil, fmt.Errorf("invalid message version tag %q; expected %q", msg.VersionTag, wireVersion)
 	}
 	id, err := MakeID(msg.ID)
 	if err != nil {