refactor(cmd_ac): DRY AI autocomplete and fix trailing space API

- Replace _ai_model_autocomplete() with new _ai_provider_and_model_autocomplete()
  that handles both provider and model autocomplete in one function
- Add /ai set default-model model autocomplete support
- Remove trailing space requirement from cmd_prefix parameter (use \"/ai start\"
  instead of \"/ai start \")
- Fix const correctness in autocomplete_param_with_func() signature
- Remove unused space_at_end variable

src/tools/autocomplete.h

@@ -63,7 +63,7 @@ gchar* autocomplete_complete(Autocomplete ac, const gchar* search_str, gboolean
 GList* autocomplete_create_list(Autocomplete ac);
 gint autocomplete_length(Autocomplete ac);
 
-char* autocomplete_param_with_func(const char* const input, char* command,
+char* autocomplete_param_with_func(const char* const input, const char* command,
                                    autocomplete_func func, gboolean previous, void* context);
 
 char* autocomplete_param_with_ac(const char* const input, char* command,