feat: built-in testing module (PyNestTestingModule + TestBed) (#118)

[ItayTheDar]

Adds nest.testing — a first-class testing toolkit modeled after @nestjs/testing and Suites:

• PyNestTestingModule.create_testing_module(imports/controllers/providers) with fluent TestingModuleBuilder
• override_provider(token).use_value/.use_class/.use_factory
• override_guard(Guard).use_value/.use_class (patches applied only during route registration, restored immediately — no state leaks between tests)
• use_auto_mock(): replaces class providers with create_autospec mocks (async methods become AsyncMocks), with exclude= and mock_factory=
• TestingModule.get/init/close, awaitable compile(), async context manager
• create_http_client() (httpx ASGITransport), create_test_client(), create_nest_application()
• TestBed.solitary/sociable with .mock().using/.final and expose() for Suites-style unit tests without a container
• PyNestContainer.replace_provider() + provider_descriptors hook
• pynest-api[testing] extra (httpx); pytest-asyncio added to test group
• docs/testing.md with pytest fixture patterns; 35 new tests

Closes#118