Instrument environment and plugin resolution paths (#33223)

## What changed

Add named tracing spans around step environment snapshots, capability and
executor plugin resolution, `AGENTS.md` refreshes, and MCP runtime projection
and refresh operations. Skip recording function arguments to keep these spans
focused on timing and execution flow.

GitOrigin-RevId: 73a452ef1b5da6f7f2b00d24421a815c5d514eaf

codex-rs/core/src/agents_md_manager.rs

@@ -28,6 +28,7 @@ impl AgentsMdManager {
         }
     }
 
+    #[tracing::instrument(name = "agents_md.refresh", skip_all)]
     pub(crate) async fn refresh(&self, config: &Config, environments: &TurnEnvironmentSnapshot) {
         let selections = environments.to_selections();
         if self.cache.lock().await.selections.as_ref() == Some(&selections) {

codex-rs/core/src/environment_selection.rs

@@ -195,6 +195,7 @@ impl ThreadEnvironments {
         .boxed()
     }
 
+    #[tracing::instrument(name = "environments.snapshot", skip_all)]
     pub(crate) async fn snapshot(&self) -> TurnEnvironmentSnapshot {
         let current = self.environments.load_full();
         let mut turn_environments = Vec::with_capacity(current.len());

codex-rs/core/src/mcp.rs

@@ -100,6 +100,7 @@ impl McpManager {
         .config
     }
 
+    #[tracing::instrument(name = "mcp.runtime_config.project_for_step", skip_all)]
     pub(crate) async fn runtime_config_for_step(
         &self,
         config: &Config,

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

@@ -109,6 +109,7 @@ impl Session {
         clippy::await_holding_invalid_type,
         reason = "MCP runtime comparison and publication must remain serialized"
     )]
+    #[tracing::instrument(name = "mcp.runtime.resolve_for_step", skip_all)]
     pub(crate) async fn mcp_runtime_for_step(
         self: &Arc<Self>,
         turn_context: &TurnContext,
@@ -319,6 +320,7 @@ impl Session {
             .await
     }
 
+    #[tracing::instrument(name = "mcp.runtime.refresh", skip_all)]
     async fn refresh_mcp_servers_inner(
         &self,
         turn_context: &TurnContext,