feat: allow failable service creation in streamable HTTP tower service (#244)

* feat: allow failable service creation in streamable HTTP tower service

In our setup we need to lookup some info that may fail before we serve
the request.

* Fix examples build

examples/servers/src/counter_hyper_streamable_http.rs

@@ -12,7 +12,7 @@ use rmcp::transport::streamable_http_server::{
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
     let service = TowerToHyperService::new(StreamableHttpService::new(
-        Counter::new,
+        || Ok(Counter::new()),
         LocalSessionManager::default().into(),
         Default::default(),
     ));

examples/servers/src/counter_streamhttp.rs

@@ -22,7 +22,7 @@ async fn main() -> anyhow::Result<()> {
         .init();
 
     let service = StreamableHttpService::new(
-        Counter::new,
+        || Ok(Counter::new()),
         LocalSessionManager::default().into(),
         Default::default(),
     );