Add spans to run_turn (#27107)

## Why
Codex app-server latency traces do not granularly cover turn
orchestration, sampling-request preparation, and tool-loading work.
These spans help separate local coordination/setup costs from model
streaming and tool execution.

## What changed
- Add `run_turn.*` spans around sampling-request input preparation and
post-sampling state collection
- Add function-level trace spans around turn setup, hook execution,
compaction, prompt construction, and MCP tool exposure
- Add `built_tools.*` spans around plugin loading and discoverable-tool
loading

## Verification
Trigger Codex rollout and observe new spans are included

codex-rs/core/src/session/mod.rs

@@ -3011,6 +3011,7 @@ impl Session {
     /// Mid-turn compaction is the other path that can re-establish that baseline when it
     /// reinjects full initial context into replacement history. Other non-regular tasks
     /// intentionally do not update the baseline.
+    #[instrument(level = "trace", skip_all)]
     pub(crate) async fn record_context_updates_and_set_reference_context_item(
         &self,
         turn_context: &TurnContext,