feat(app-server, core): add more spans (#14479)

## Description

This PR expands tracing coverage across app-server thread startup, core
session initialization, and the Responses transport layer. It also gives
core dispatch spans stable operation-specific names so traces are easier
to follow than the old generic `submission_dispatch` spans.

Also use `fmt::Display` for types that we serialize in traces so we send
strings instead of rust types

codex-rs/codex-api/src/endpoint/session.rs

@@ -12,6 +12,7 @@ use http::HeaderMap;
 use http::Method;
 use serde_json::Value;
 use std::sync::Arc;
+use tracing::instrument;
 
 pub(crate) struct EndpointSession<T: HttpTransport, A: AuthProvider> {
     transport: T,
@@ -68,6 +69,12 @@ impl<T: HttpTransport, A: AuthProvider> EndpointSession<T, A> {
             .await
     }
 
+    #[instrument(
+        name = "endpoint_session.execute_with",
+        level = "info",
+        skip_all,
+        fields(http.method = %method, api.path = path)
+    )]
     pub(crate) async fn execute_with<C>(
         &self,
         method: Method,
@@ -96,6 +103,12 @@ impl<T: HttpTransport, A: AuthProvider> EndpointSession<T, A> {
         Ok(response)
     }
 
+    #[instrument(
+        name = "endpoint_session.stream_with",
+        level = "info",
+        skip_all,
+        fields(http.method = %method, api.path = path)
+    )]
     pub(crate) async fn stream_with<C>(
         &self,
         method: Method,