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>

mcp/testdata/conformance/server/resources.txtar

@@ -5,6 +5,7 @@ Fixed bugs:
 
 -- resources --
 info
+info.txt
 
 -- client --
 {
@@ -25,6 +26,19 @@ info
     "uri": "embedded:info"
   }
 }
+{
+  "jsonrpc": "2.0", "id": 3,
+  "method": "resources/read",
+  "params": {
+    "uri": "file:///info.txt"
+  }
+}
+{
+  "jsonrpc": "2.0", "id": 0,
+  "result": {
+	"roots": []
+  }
+}
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -61,6 +75,11 @@ info
 				"mimeType": "text/plain",
 				"name": "info",
 				"uri": "embedded:info"
+			},
+			{
+				"mimeType": "text/plain",
+				"name": "public",
+				"uri": "file:///info.txt"
 			}
 		]
 	}
@@ -79,3 +98,23 @@ info
 		]
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"method": "roots/list",
+	"params": null
+}
+{
+	"jsonrpc": "2.0",
+	"id": 3,
+	"result": {
+		"_meta": {},
+		"contents": [
+			{
+				"uri": "file:///info.txt",
+				"mimeType": "text/plain",
+				"blob": "Q29udGVudHMK"
+			}
+		]
+	}
+}