Add deferred MCP reprojection for the next regular turn (#61356)

## What changed

Add `CodexThread::reproject_mcp_on_next_turn()` to request an MCP reprojection after `TurnStarted` and before tools are bound. Repeated requests coalesce, and idle reconciliation or prewarming leaves the request pending until the next regular turn.

## Testing

Add coverage for deferred requests surviving prewarm and idle reconciliation, coalescing into one reprojection, and exposing refreshed tools in the turn's first model request. Verify that discovery starts after `TurnStarted`.

GitOrigin-RevId: 260b2381013291a47eb9a7f736c129de76c5897f

codex-rs/core/src/codex_thread.rs

@@ -1276,6 +1276,12 @@ impl CodexThread {
         self.session.hooks().list_with_environment_hooks(sources)
     }
 
+    /// Rebuilds the MCP projection on the next regular turn, after turn start and before
+    /// tools are bound. Repeated requests coalesce without starting a background refresh.
+    pub fn reproject_mcp_on_next_turn(&self) {
+        self.session.reproject_mcp_on_next_turn();
+    }
+
     /// Observes this thread's published MCP connections that match the requested config.
     pub async fn mcp_connection_statuses(
         &self,