Record metrics for empty skill catalogs (#35607)

## Why

Empty rendered skill catalogs skipped catalog telemetry instead of reporting
zero-valued counts.

## What changed

- Record a default render report when thread, turn-input, or executor catalog
  rendering produces no fragment.
- Record zero-valued host world-state metrics when skill instructions are
  enabled but no host skills are available.
- Preserve the `catalog_surface` tag so empty host and executor catalogs remain
  distinguishable.

## Testing

Add coverage for empty catalog rendering and mixed nonempty-executor,
empty-host world state.

GitOrigin-RevId: f68d65806703c0ded1ece1e5ae1ffaf6d6045126

codex-rs/ext/skills/src/render.rs

@@ -88,7 +88,7 @@ pub(crate) enum SkillMetadataBudget {
     Characters(usize),
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
 pub(crate) struct SkillRenderReport {
     pub(crate) total_count: usize,
     pub(crate) included_count: usize,