Skip removed features during metrics emission (#12253)

Summary
- avoid emitting metrics for features marked as `Stage::Removed`
- keep feature metrics aligned with active and planned states only

Testing
- Not run (not requested)

codex-rs/core/src/features.rs

@@ -250,6 +250,9 @@ impl Features {
 
     pub fn emit_metrics(&self, otel: &OtelManager) {
         for feature in FEATURES {
+            if matches!(feature.stage, Stage::Removed) {
+                continue;
+            }
             if self.enabled(feature.id) != feature.default_enabled {
                 otel.counter(
                     "codex.feature.state",