[codex] add latency tracing spans (#27710)

## Why

We have some large gaps in our thread start, resume, and pre-sampling
traces that make it hard to tell where latency is coming from.

## What Changed

- Added coarse spans around thread start/resume, turn context
construction, rollout reconstruction, skill/plugin loading, and tool
preparation.
- Added a breakdown of discoverable-tool preparation across connector
loading, plugin discovery, and local plugin details.

## Testing

- `cargo check -p codex-app-server -p codex-core -p codex-core-skills -p
codex-core-plugins`
- Built the app-server locally and exercised thread start, first turn,
follow-up turn, server restart, thread resume, and a resumed turn.

codex-rs/core/src/connectors.rs

@@ -18,6 +18,7 @@ use codex_tools::DiscoverableTool;
 use rmcp::model::ToolAnnotations;
 use serde::Deserialize;
 use tokio_util::sync::CancellationToken;
+use tracing::instrument;
 use tracing::warn;
 
 use crate::config::Config;
@@ -460,6 +461,7 @@ fn tool_suggest_connector_ids(
     connector_ids
 }
 
+#[instrument(level = "trace", skip_all)]
 async fn cached_directory_connectors_for_tool_suggest_with_auth(
     config: &Config,
     auth: Option<&CodexAuth>,