fix: align skip-guard tests with is_backend_ready and drop unused import

tests/_backend.py imported is_backend_available but never called it,
causing a pyright reportUnusedImport error. Remove the unused import.

test_backend_skip_guards.py was patching tests._backend.has_backend but
_backend.py calls is_backend_ready — the mock never intercepted the real
call, producing AttributeError at test runtime. Update all patch targets
to is_backend_ready and add quiet=False to the assert_called_once_with
assertion to match the actual call signature.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/_backend.py

@@ -13,7 +13,7 @@ from __future__ import annotations
 import os
 import unittest
 
-from bot_bottle.backend import is_backend_available, is_backend_ready
+from bot_bottle.backend import is_backend_ready
 
 # Default when ``BOT_BOTTLE_BACKEND`` is unset. Docker preserves the historical
 # Docker-backed CI path (and mirrors the pin in ``test_sandbox_escape``).