Add compatibility policies for skill catalog rendering (#34611)

## What changed

- Parameterize skill catalog rendering with explicit core- and extension-compatible description policies.
- Use full skill descriptions for core-compatible output, while extension-compatible output prefers `short_description` and falls back to the full description.
- Keep extension prompt and world-state rendering on the extension-compatible policy.

## Testing

- Add a renderer test covering description selection and fallback for both policies.

GitOrigin-RevId: 5382075f88c25a49a715feb0702a9ab888ec0123

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

@@ -0,0 +1,67 @@
+use super::*;
+use crate::catalog::SkillAuthority;
+use crate::catalog::SkillPackageId;
+use crate::catalog::SkillResourceId;
+use codex_core_skills::render_available_skills_body;
+use codex_extension_api::ContextualUserFragment;
+use pretty_assertions::assert_eq;
+
+fn entry(name: &str, description: &str, short_description: Option<&str>) -> SkillCatalogEntry {
+    SkillCatalogEntry::new(
+        SkillPackageId(name.to_string()),
+        SkillAuthority::new(SkillSourceKind::Host, "host"),
+        name,
+        description,
+        SkillResourceId::new(format!("/skills/{name}/SKILL.md")),
+    )
+    .with_short_description(short_description.map(str::to_string))
+}
+
+#[test]
+fn description_selection_follows_render_policy() {
+    let catalog = SkillCatalog {
+        entries: vec![
+            entry("shortened", "full description", Some("short description")),
+            entry(
+                "fallback",
+                "fallback description",
+                /*short_description*/ None,
+            ),
+        ],
+        warnings: Vec::new(),
+    };
+
+    let core = available_skills_fragment(
+        &catalog,
+        /*include_skills_usage_instructions*/ false,
+        SkillCatalogRenderPolicy::CoreCompatible,
+    )
+    .expect("catalog should render");
+    let extension = available_skills_fragment(
+        &catalog,
+        /*include_skills_usage_instructions*/ false,
+        SkillCatalogRenderPolicy::ExtensionCompatible,
+    )
+    .expect("catalog should render");
+
+    assert_eq!(
+        core.body(),
+        render_available_skills_body(
+            &[],
+            &[
+                "- shortened: full description (file: /skills/shortened/SKILL.md)".to_string(),
+                "- fallback: fallback description (file: /skills/fallback/SKILL.md)".to_string(),
+            ],
+        )
+    );
+    assert_eq!(
+        extension.body(),
+        render_available_skills_body(
+            &[],
+            &[
+                "- shortened: short description (file: /skills/shortened/SKILL.md)".to_string(),
+                "- fallback: fallback description (file: /skills/fallback/SKILL.md)".to_string(),
+            ],
+        )
+    );
+}