fix: address Copilot review comments on type safety and test API

- Make is_slash_skills_agent() accept str | None to match its call sites
  (init_options.get("ai") can return None)
- Refactor TestSlashSkillsSets to use public execute_hook() API instead of
  private _render_hook_invocation() method

Author: arrrrny

src/specify_cli/extensions.py

@@ -52,7 +52,7 @@
 EXTENSION_COMMAND_NAME_PATTERN = re.compile(r"^speckit\.([a-z0-9-]+)\.([a-z0-9-]+)$")
 
 
-def is_slash_skills_agent(selected_ai: str, ai_skills_enabled: bool) -> bool:
+def is_slash_skills_agent(selected_ai: str | None, ai_skills_enabled: bool) -> bool:
     """Return True if *selected_ai* uses ``/speckit-<name>`` hook invocations.
 
     The decision is based on the agent sets defined above:
@@ -62,6 +62,8 @@ def is_slash_skills_agent(selected_ai: str, ai_skills_enabled: bool) -> bool:
         *ai_skills_enabled* is ``True``.
     *   All other agents return ``False``.
     """
+    if selected_ai is None:
+        return False
     return selected_ai in ALWAYS_SLASH_AGENTS or (
         selected_ai in CONDITIONAL_SLASH_AGENTS and ai_skills_enabled
     )

tests/integrations/test_integration_zed.py

@@ -125,7 +125,10 @@ def _render_invocation(project_path, ai: str, ai_skills: bool) -> str:
         init_options.parent.mkdir(parents=True, exist_ok=True)
         init_options.write_text(json.dumps({"ai": ai, "ai_skills": ai_skills}))
         hook_executor = HookExecutor(project_path)
-        return hook_executor._render_hook_invocation("speckit.plan")
+        result = hook_executor.execute_hook(
+            {"extension": "test-ext", "command": "speckit.plan", "optional": False}
+        )
+        return result.get("invocation", "")
 
     @pytest.mark.parametrize(
         ("ai", "ai_skills", "expected"),