feat!: add server discovery and negotiation (SEP-2575) (#973)

* feat!: add server discovery and negotiation

* feat!: make protocol unions extensible

* ci: add discovery conformance coverage

crates/rmcp/tests/test_server_discover_client.rs

@@ -0,0 +1,77 @@
+#![cfg(all(feature = "client", not(feature = "local")))]
+
+use rmcp::{
+    ClientHandler, ServerHandler, ServiceExt,
+    model::{
+        ClientCapabilities, Implementation, ProtocolVersion, RequestMetaObject, ServerCapabilities,
+        ServerInfo,
+    },
+    select_protocol_version,
+};
+
+#[derive(Clone, Default)]
+struct DiscoveryServer;
+
+impl ServerHandler for DiscoveryServer {
+    fn get_info(&self) -> ServerInfo {
+        ServerInfo::new(ServerCapabilities::builder().enable_tools().build())
+            .with_server_info(Implementation::new("discovery-server", "1.0.0"))
+    }
+}
+
+#[derive(Clone, Default)]
+struct DiscoveryClient;
+
+impl ClientHandler for DiscoveryClient {}
+
+#[test]
+fn select_protocol_version_uses_client_preference_order() {
+    let selected = select_protocol_version(
+        &[ProtocolVersion::V_2026_07_28, ProtocolVersion::V_2025_11_25],
+        &[ProtocolVersion::V_2025_11_25, ProtocolVersion::V_2026_07_28],
+    );
+
+    assert_eq!(selected, Some(ProtocolVersion::V_2026_07_28));
+}
+
+#[test]
+fn select_protocol_version_returns_none_without_overlap() {
+    let selected = select_protocol_version(
+        &[ProtocolVersion::V_2026_07_28],
+        &[ProtocolVersion::V_2025_11_25],
+    );
+
+    assert_eq!(selected, None);
+}
+
+#[tokio::test]
+async fn client_discover_helper_returns_typed_result() {
+    let (server_transport, client_transport) = tokio::io::duplex(4096);
+    tokio::spawn(async move {
+        let _ = DiscoveryServer
+            .serve(server_transport)
+            .await
+            .expect("server should start")
+            .waiting()
+            .await;
+    });
+    let client = DiscoveryClient
+        .serve(client_transport)
+        .await
+        .expect("client should connect");
+    let mut meta = RequestMetaObject::new();
+    meta.set_protocol_version(ProtocolVersion::V_2026_07_28);
+    meta.set_client_info(Implementation::new("discovery-client", "1.0.0"));
+    meta.set_client_capabilities(ClientCapabilities::default());
+
+    let result = client
+        .discover(meta)
+        .await
+        .expect("discover should succeed");
+
+    assert_eq!(
+        result.server_info,
+        Implementation::new("discovery-server", "1.0.0")
+    );
+    client.cancel().await.expect("client should cancel");
+}