[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/session/turn_context.rs

@@ -20,6 +20,7 @@ use codex_sandboxing::policy_transforms::effective_network_sandbox_policy;
 use codex_utils_path_uri::PathUri;
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
+use tracing::instrument;
 
 #[derive(Clone, Debug)]
 pub(crate) struct TurnSkillsContext {
@@ -724,6 +725,7 @@ impl Session {
         .await
     }
 
+    #[instrument(name = "turn_context.build", level = "trace", skip_all)]
     async fn new_turn_context_from_configuration(
         &self,
         sub_id: String,