Adds a BDD scenario tagged @tdd_issue @tdd_issue_4750 @tdd_expected_fail
that captures the bug: when only GEMINI_API_KEY is set (without GOOGLE_API_KEY),
ProviderRegistry.get_default_provider_type() returns None instead of
ProviderType.GEMINI because GEMINI is absent from FALLBACK_ORDER.
The @tdd_expected_fail tag inverts the result so CI passes while the bug exists.
Once the fix is applied, the tag must be removed.
ISSUES CLOSED: #10896
HAL9000