[codex] trace MCP startup latency (#28630)

## Summary

- add trace-level instrumentation around per-server MCP setup, client
construction, initialization, and initial tool listing
- trace Codex Apps tool and server-info cache loads
- attach `server_name` to server-scoped spans so slow startup work can
be attributed to a specific MCP server

## Why

`session_init.mcp_manager_init` can occasionally be slow, but its
existing coarse span does not identify whether time is spent loading the
Codex Apps cache, constructing a client, initializing a transport, or
listing tools. These definition-level spans provide that breakdown
without changing startup behavior.

## Validation

- `just test -p codex-mcp` (87 passed)
- `just test -p codex-rmcp-client` (86 passed, 2 skipped)

codex-rs/rmcp-client/src/rmcp_client.rs

@@ -58,6 +58,7 @@ use tokio::sync::Mutex;
 use tokio::sync::Semaphore;
 use tokio::sync::watch;
 use tokio::time;
+use tracing::instrument;
 use tracing::warn;
 
 use crate::elicitation_client_service::ElicitationClientService;
@@ -420,6 +421,7 @@ impl RmcpClient {
 
     /// Perform the initialization handshake with the MCP server.
     /// https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle#initialization
+    #[instrument(level = "trace", skip_all)]
     pub async fn initialize(
         &self,
         params: InitializeRequestParams,
@@ -501,6 +503,7 @@ impl RmcpClient {
         Ok(result)
     }
 
+    #[instrument(level = "trace", skip_all)]
     pub async fn list_tools_with_connector_ids(
         &self,
         params: Option<PaginatedRequestParams>,