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-protocol/src/jsonrpc_lite.rs

@@ -5,6 +5,7 @@ use codex_protocol::protocol::W3cTraceContext;
 use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
+use std::fmt;
 use ts_rs::TS;
 
 pub const JSONRPC_VERSION: &str = "2.0";
@@ -19,6 +20,15 @@ pub enum RequestId {
     Integer(i64),
 }
 
+impl fmt::Display for RequestId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::String(value) => f.write_str(value),
+            Self::Integer(value) => write!(f, "{value}"),
+        }
+    }
+}
+
 pub type Result = serde_json::Value;
 
 /// Refers to any valid JSON-RPC object that can be decoded off the wire, or encoded to be sent.