internal/mcp: add resource conformance test

This uncovered a problem in how we represent resource results:
contents is a slice, not a single value.

I tried to use a file resource, but the server->client ListRoots
call didn't work because synthetic responses aren't supported.
Left a TODO.

I enhanced the hello example so I that could use the MCP Inspector to
work with resources.

The falseSchema change fixes a potential bug where schemas are
re-used, which is disallowed.

Change-Id: I1f0198043cb6221c2a03f4f83aa0bafbc71167f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/677136
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>

mcp/testdata/conformance/server/resources.txtar

@@ -0,0 +1,81 @@
+Check behavior of a server with just resources.
+
+Fixed bugs:
+- A resource result holds a slice of contents, not just one.
+
+-- resources --
+info
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2024-11-05",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+{ "jsonrpc": "2.0", "id": 2, "method": "resources/list" }
+{
+  "jsonrpc": "2.0", "id": 3,
+  "method": "resources/read",
+  "params": {
+    "uri": "embedded:info"
+  }
+}
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"result": {
+		"_meta": {},
+		"capabilities": {
+			"completions": {},
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			},
+			"resources": {
+				"listChanged": true
+			},
+			"tools": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2024-11-05",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0"
+		}
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 2,
+	"result": {
+		"_meta": {},
+		"resources": [
+			{
+				"mimeType": "text/plain",
+				"name": "info",
+				"uri": "embedded:info"
+			}
+		]
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 3,
+	"result": {
+		"_meta": {},
+		"contents": [
+			{
+				"uri": "embedded:info",
+				"mimeType": "text/plain",
+				"text": "This is the MCP test server."
+			}
+		]
+	}
+}