Add metrics for extension-rendered skill catalogs (#35597)

## What changed

- Add an optional `ExtensionMetrics` capability to thread, turn-input, and world-state contributors, backed by session telemetry in the host.
- Record skill counts, omissions, and truncated description characters for extension-rendered catalogs, tagged by the catalog surface.
- Preserve host session attribution on extension metrics, including the effective model for each turn, and prevent extension tags from overriding host metadata.
- Avoid duplicate host-catalog samples by recording world-state metrics only when the section is published or changes.

## Testing

- Cover metric values and surface tags, session metadata preservation, turn-level model changes, and host world-state publication behavior.

GitOrigin-RevId: 9059e521943a8f81df25a38b03692eddbc1d990d

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

@@ -121,7 +121,7 @@ impl SkillRenderReport {
             .then(|| SKILL_DESCRIPTION_TRUNCATED_WARNING.to_string())
     }
 
-    fn average_truncated_description_chars(&self) -> usize {
+    pub(crate) fn average_truncated_description_chars(&self) -> usize {
         if self.total_count == 0 || self.truncated_description_chars == 0 {
             return 0;
         }
@@ -158,7 +158,7 @@ fn metadata_line_cost(budget: SkillMetadataBudget, line: &str) -> usize {
 }
 
 impl SkillMetadataBudget {
-    fn limit(self) -> usize {
+    pub(crate) fn limit(self) -> usize {
         match self {
             Self::Tokens(limit) | Self::Characters(limit) => limit,
         }