Add fake camera SDKs and contract tests

Introduce comprehensive test scaffolding for camera backends: add fake SDK implementations and patching fixtures in tests/cameras/backends/conftest.py (FakeAravis, FakePylon, FakeHarvester and helpers to monkeypatch aravis/pypylon/harvesters). Add backend-agnostic contract tests in tests/cameras/backends/test_generic_contracts.py to validate backend capability shapes, availability reporting, safe discovery, create/close semantics, optional accelerator warnings, and a GUI identity helper. These changes enable running backend contract tests in CI without real SDKs and help ensure consistent backend behavior.

Author: C-Achard