handlers: session_or_hydrate returns the dashmap guard, not Arc

the session index holds values; get() hands back a Ref tied to the map
borrow. The Arc-typed helper was a type error on its first CI run.

src/api/handlers.rs

@@ -4,10 +4,11 @@ use crate::server::SouveraineServer;
 /// Resolve a session, hydrating from disk when a freshly booted server has
 /// not loaded this conversation yet. Surfaces hold conversation ids, not a
 /// map; a restart must answer a held id without a prior listing.
-async fn session_or_hydrate(
-    server: &SouveraineServer,
+async fn session_or_hydrate<'a>(
+    server: &'a SouveraineServer,
     id: &str,
-) -> Result<Arc<crate::server::session_manager::Session>, ApiError> {
+) -> Result<dashmap::mapref::one::Ref<'a, String, crate::server::session_manager::Session>, ApiError>
+{
     if let Some(session) = server.sessions.get(id) {
         return Ok(session);
     }