fix(ai): use atomic ai_provider_ref() in cmd_ai_switch

src/command/cmd_funcs:11118 used a plain non-atomic ref_count++ on
ai_provider, which is a data race against concurrent
ai_provider_unref() calls using g_atomic_int_dec_and_test.

- Expose ai_provider_ref() in ai_client.h (was static)
- Replace non-atomic ref_count++ with ai_provider_ref() in cmd_ai_switch"

src/ai/ai_client.h

@@ -78,6 +78,13 @@ AIProvider* ai_add_provider(const gchar* name, const gchar* api_url, const gchar
  */
 gboolean ai_remove_provider(const gchar* name);
 
+/**
+ * Increment the reference count of a provider.
+ * @param provider The provider to reference
+ * @return The same provider pointer
+ */
+AIProvider* ai_provider_ref(AIProvider* provider);
+
 /**
  * Decrement the reference count of a provider.
  * @param provider The provider to unreference