fix(providers): add missing ProviderType.GEMINI step definition for fallback order test

Add @then(the provider registry result should be ProviderType.GEMINI) step
definition to features/steps/provider_registry_steps.py. This step was referenced
by the new Gemini-only user gets GEMINI as default provider via fallback order
scenario in provider_registry_coverage.feature but had no corresponding
implementation, causing the unit_tests CI gate to fail.

The new step follows the same pattern as the existing ProviderType.OPENAI and
ProviderType.ANTHROPIC result steps.

features/steps/provider_registry_steps.py

@@ -728,6 +728,11 @@ def step_impl_result_provider_type_anthropic(context: Any) -> None:
     assert context.result == _ensure_provider_type(context).ANTHROPIC
 
 
+@then("the provider registry result should be ProviderType.GEMINI")
+def step_impl_result_provider_type_gemini(context: Any) -> None:
+    assert context.result == _ensure_provider_type(context).GEMINI
+
+
 @then('the provider registry result should be "{value}"')
 def step_impl_result_string(context: Any, value: str) -> None:
     assert context.result == value, f"Expected {value!r}, got {context.result!r}"