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/app-server/src/app_server_tracing.rs

@@ -92,7 +92,7 @@ fn transport_name(transport: AppServerTransport) -> &'static str {
 fn app_server_request_span_template(
     method: &str,
     transport: &'static str,
-    request_id: &impl std::fmt::Debug,
+    request_id: &impl std::fmt::Display,
     connection_id: ConnectionId,
 ) -> Span {
     info_span!(
@@ -102,8 +102,8 @@ fn app_server_request_span_template(
         rpc.system = "jsonrpc",
         rpc.method = method,
         rpc.transport = transport,
-        rpc.request_id = ?request_id,
-        app_server.connection_id = ?connection_id,
+        rpc.request_id = %request_id,
+        app_server.connection_id = %connection_id,
         app_server.api_version = "v2",
         app_server.client_name = field::Empty,
         app_server.client_version = field::Empty,
@@ -122,14 +122,14 @@ fn record_client_info(span: &Span, client_name: Option<&str>, client_version: Op
 fn attach_parent_context(
     span: &Span,
     method: &str,
-    request_id: &impl std::fmt::Debug,
+    request_id: &impl std::fmt::Display,
     parent_trace: Option<&W3cTraceContext>,
 ) {
     if let Some(trace) = parent_trace {
         if !set_parent_from_w3c_trace_context(span, trace) {
             tracing::warn!(
                 rpc_method = method,
-                rpc_request_id = ?request_id,
+                rpc_request_id = %request_id,
                 "ignoring invalid inbound request trace carrier"
             );
         }