The test 'hephaestus is created when github-copilot provider is connected'
had incorrect expectation. github-copilot does not provide gpt-5.3-codex,
so hephaestus should NOT be created when only github-copilot is connected.
This test was causing CI flakiness due to incorrect assertion and
missing readConnectedProvidersCache mock (state pollution between tests).
Also adds cacheSpy mock for proper isolation.