[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/plugins/discoverable.rs

@@ -5,7 +5,9 @@ use codex_core_plugins::ToolSuggestPluginDiscoveryInput;
 use codex_login::CodexAuth;
 use codex_tools::DiscoverablePluginInfo;
 use std::collections::HashSet;
+use tracing::instrument;
 
+#[instrument(level = "trace", skip_all)]
 pub(crate) async fn list_tool_suggest_discoverable_plugins(
     config: &Config,
     plugins_manager: &PluginsManager,