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.c

@@ -305,7 +305,7 @@ autocomplete_complete(Autocomplete ac, const gchar* search_str, gboolean quote,
 // autocomplete_func func is used -> autocomplete_param_with_func
 // Autocomplete ac, gboolean quote are used -> autocomplete_param_with_ac
 static char*
-_autocomplete_param_common(const char* const input, char* command, autocomplete_func func, Autocomplete ac, gboolean quote, gboolean previous, void* context)
+_autocomplete_param_common(const char* const input, const char* command, autocomplete_func func, Autocomplete ac, gboolean quote, gboolean previous, void* context)
 {
     int len;
     auto_gchar gchar* command_cpy = g_strdup_printf("%s ", command);
@@ -344,7 +344,7 @@ _autocomplete_param_common(const char* const input, char* command, autocomplete_
 }
 
 char*
-autocomplete_param_with_func(const char* const input, char* command, autocomplete_func func, gboolean previous, void* context)
+autocomplete_param_with_func(const char* const input, const char* command, autocomplete_func func, gboolean previous, void* context)
 {
     return _autocomplete_param_common(input, command, func, NULL, FALSE, previous, context);
 }