chore: add #[non_exhaustive] and mutation methods to improve compatibility (#715)

* chore: add #[non_exhaustive] to reduce backwards-incompatible changes going forward

* fix: remove ProtocolVersion import

* fix: add a few more with_ mutator methods

---------

Co-authored-by: Alex Hancock <alexhancock@block.xyz>

crates/rmcp/tests/test_custom_headers.rs

@@ -730,10 +730,7 @@ async fn test_server_rejects_unsupported_protocol_version() {
 
     impl ServerHandler for TestHandler {
         fn get_info(&self) -> ServerInfo {
-            ServerInfo {
-                capabilities: ServerCapabilities::builder().build(),
-                ..Default::default()
-            }
+            ServerInfo::new(ServerCapabilities::builder().build())
         }
     }