mcp: update return meta in DiscoverResult (SEP-2575) (#1097)

This PR implements the latest changes introduced by
https://github.com/modelcontextprotocol/modelcontextprotocol/pull/3002

mcp/testdata/conformance/server/discover.txtar

@@ -1,7 +1,7 @@
-The client sends a discover request
-carrying the required per-request _meta triple (protocolVersion, clientInfo,
-clientCapabilities), and the server replies with the protocol versions it
-supports, its capabilities, and its identity.
+The client sends a discover request carrying the per-request _meta
+(protocolVersion, clientInfo, clientCapabilities), and the server
+replies with the protocol versions it supports, its capabilities, and its
+identity via `_meta.io.modelcontextprotocol/serverInfo`.
 
 -- client --
 {
@@ -22,6 +22,12 @@ supports, its capabilities, and its identity.
 	"id": 1,
 	"result": {
 		"resultType": "complete",
+		"_meta": {
+			"io.modelcontextprotocol/serverInfo": {
+				"name": "testServer",
+				"version": "v1.0.0"
+			}
+		},
 		"ttlMs": 0,
 		"cacheScope": "public",
 		"supportedVersions": [
@@ -33,10 +39,6 @@ supports, its capabilities, and its identity.
 		],
 		"capabilities": {
 			"logging": {}
-		},
-		"serverInfo": {
-			"name": "testServer",
-			"version": "v1.0.0"
 		}
 	}
 }

mcp/testdata/conformance/server/mrtr.txtar

@@ -45,6 +45,12 @@ confirmThenGreet
 	"jsonrpc": "2.0",
 	"id": 1,
 	"result": {
+		"_meta": {
+			"io.modelcontextprotocol/serverInfo": {
+				"name": "testServer",
+				"version": "v1.0.0"
+			}
+		},
 		"content": null,
 		"requestState": "step=1",
 		"resultType": "input_required",
@@ -71,6 +77,12 @@ confirmThenGreet
 	"jsonrpc": "2.0",
 	"id": 2,
 	"result": {
+		"_meta": {
+			"io.modelcontextprotocol/serverInfo": {
+				"name": "testServer",
+				"version": "v1.0.0"
+			}
+		},
 		"content": [
 			{
 				"type": "text",