Align SEP-2575 checks with spec PR #3002: serverInfo in result _meta, clientInfo optional (#403)

Spec PR #3002 (part of the final 2026-07-28 revision) moved serverInfo from
the DiscoverResult body to _meta['io.modelcontextprotocol/serverInfo'] and
demoted the request envelope's clientInfo from required to SHOULD. The
checks still asserted the old shape, failing SDKs that follow the final
revision.

- Vendor schema types at spec commit 71e30695.
- server-stateless: drop the missing-clientInfo rejection case and assert
  the opposite (a clientInfo-less request must be served, new check
  sep-2575-request-meta-client-info-optional); discover no longer requires
  body serverInfo; new SHOULD-level check
  sep-2575-server-identifies-in-result-meta (WARNING when the _meta
  identity is absent, SKIPPED when discover itself failed).
- request-metadata (client side): the clientInfo SHOULD moves out of the
  MUST check into its own id sep-2575-client-sends-client-info (WARNING on
  absence), so requirement levels never blend in one check id.
- Mock servers and the everything-server example emit the _meta identity
  and accept clientInfo-less requests.

examples/servers/typescript/everything-server.ts

@@ -1266,11 +1266,11 @@ app.post('/mcp', async (req, res) => {
 
     // Per-Request Metadata Integrity Checks (Fields verification).
     // A request missing any required `_meta` field is malformed: -32602 and,
-    // on HTTP, status 400 Bad Request.
+    // on HTTP, status 400 Bad Request. `clientInfo` is a SHOULD since spec
+    // PR #3002 and is never required.
     if (
       !meta ||
       !meta['io.modelcontextprotocol/protocolVersion'] ||
-      !meta['io.modelcontextprotocol/clientInfo'] ||
       !meta['io.modelcontextprotocol/clientCapabilities']
     ) {
       return res.status(400).json({
@@ -1372,7 +1372,13 @@ app.post('/mcp', async (req, res) => {
             // served on this path, so the capability must be declared too.
             resources: {}
           },
-          serverInfo: { name: 'everything-stateless-server', version: '1.0.0' }
+          // Spec PR #3002: server identity lives in the result `_meta`.
+          _meta: {
+            'io.modelcontextprotocol/serverInfo': {
+              name: 'everything-stateless-server',
+              version: '1.0.0'
+            }
+          }
         }
       });
     }