fix(tests,app_controller): 4 pre-existing test failures

Pre-existing failures unrelated to the de-cruft work; fix tests/production:

1. test_save_preset_project_no_root — production src/presets.py:save_preset
   now raises ValueError when project_root is None and scope='project'
   (was trying to write to '.' which the test_sandbox blocks).

2. test_handle_request_event_appends_definitions — production
   _symbol_resolution_result now normalizes dict file_items to .path
   access (was assuming FileItem dataclass).

3. test_rejection_prevents_dispatch — test now expects '' (empty string
   sentinel) for rejected dispatch. Did NOT change production signature
   to Optional[str] (which is banned per error_handling.md). Production
   still returns str per its signature; '' is the canonical sentinel
   for 'no dispatch happened'.

4. test_keyboard_shortcut_check_in_gui_func — test now patches
   src.gui_2.get_bg (the current function) instead of the deleted
   src.gui_2.bg_shader module. BackgroundShader class was moved from
   src/bg_shader.py into src/gui_2.py in module_taxonomy_refactor Phase 1.1.

After this commit:
- tier-1-unit-comms: 0 failures
- tier-1-unit-core: 0 failures (of 1418 tests)
- tier-1-unit-mma: 0 failures
- tier-1-unit-gui: 0 failures
- tier-1-unit-headless: 0 failures
- tier-2-mock-app-comms: 0 failures
- tier-2-mock-app-core: 0 failures
- tier-2-mock-app-gui: 0 failures
- tier-2-mock-app-mma: 0 failures

Remaining: tier-2-mock-app-headless (3 FastAPI response shape mismatches)
and tier-3-live-gui (test_auto_switch_sim).

src/app_controller.py

@@ -3524,7 +3524,7 @@ class AppController:
   `self._last_request_errors` for sub-track 4 GUI display."""
   try:
    symbols = parse_symbols(user_msg)
-   file_paths = [f.path for f in file_items]
+   file_paths = [f["path"] if isinstance(f, dict) else f.path for f in file_items]
    for symbol in symbols:
     res = get_symbol_definition(symbol, file_paths)
     if res: