fix(providers): add ProviderType.GEMINI to ProviderRegistry.FALLBACK_ORDER

ProviderType.GEMINI was missing from FALLBACK_ORDER, causing users who
configure only GEMINI_API_KEY (without GOOGLE_API_KEY) to never have
Gemini auto-selected as the default provider. get_default_provider_type()
would return None instead of ProviderType.GEMINI for Gemini-only setups.

Fix: add ProviderType.GEMINI to FALLBACK_ORDER after ProviderType.GOOGLE,
since GEMINI and GOOGLE share the same underlying model family but use
different API keys (GEMINI_API_KEY vs GOOGLE_API_KEY).

Also adds:
- TDD regression test (tdd_gemini_fallback_order_4750.feature) with
  @tdd_issue @tdd_issue_4750 tags (without @tdd_expected_fail since fix applied)
- New BDD scenarios in provider_registry_coverage.feature verifying GEMINI
  is in FALLBACK_ORDER and that Gemini-only users get GEMINI as default
- New step definitions for GEMINI fallback order verification

ISSUES CLOSED: #4750

features/provider_registry_coverage.feature

@@ -466,3 +466,17 @@ Feature: Provider Registry Coverage
     And Anthropic default should be "claude-sonnet-4-20250514"
     And Google default should be "gemini-2.0-flash"
     And Groq default should be "llama-3.1-70b-versatile"
+
+  @unit @providers @registry
+  Scenario: GEMINI is included in provider fallback order
+    Given I have the ProviderRegistry class
+    When I check the FALLBACK_ORDER
+    Then GEMINI should be in the fallback order
+    And GEMINI should appear after GOOGLE in the fallback order
+
+  @unit @providers @registry
+  Scenario: Gemini-only user gets GEMINI as default provider via fallback order
+    Given I have a ProviderRegistry with "gemini" API key set
+    And CLEVERAGENTS_DEFAULT_PROVIDER is not set
+    When I call get_default_provider_type
+    Then the provider registry result should be ProviderType.GEMINI

features/steps/provider_registry_steps.py

@@ -922,3 +922,23 @@ def step_impl_default_models(context: Any, provider: str, model: str) -> None:
 # -- Hooks ------------------------------------------------------------------
 
 # Hook logic lives in features/environment.py to ensure scenarios reset state.
+
+
+@then("GEMINI should be in the fallback order")
+def step_impl_gemini_in_fallback_order(context: Any) -> None:
+    provider_enum = _ensure_provider_type(context)
+    assert provider_enum.GEMINI in context.fallback_order, (
+        f"Expected ProviderType.GEMINI to be in FALLBACK_ORDER but it was not found. "
+        f"FALLBACK_ORDER: {context.fallback_order}"
+    )
+
+
+@then("GEMINI should appear after GOOGLE in the fallback order")
+def step_impl_gemini_after_google(context: Any) -> None:
+    provider_enum = _ensure_provider_type(context)
+    google_idx = context.fallback_order.index(provider_enum.GOOGLE)
+    gemini_idx = context.fallback_order.index(provider_enum.GEMINI)
+    assert gemini_idx > google_idx, (
+        f"Expected ProviderType.GEMINI (index {gemini_idx}) to appear after "
+        f"ProviderType.GOOGLE (index {google_idx}) in FALLBACK_ORDER."
+    )

features/steps/tdd_gemini_fallback_order_4750_steps.py

@@ -0,0 +1,68 @@
+"""Step definitions for TDD test: ProviderType.GEMINI missing from FALLBACK_ORDER.
+
+Issue: #4750 — ProviderType.GEMINI missing from ProviderRegistry.FALLBACK_ORDER
+TDD Issue: #10896
+
+This test captures the bug: when only GEMINI_API_KEY is set (without GOOGLE_API_KEY),
+get_default_provider_type() returns None instead of ProviderType.GEMINI because
+GEMINI is absent from FALLBACK_ORDER.
+
+The @tdd_expected_fail tag has been removed because the fix has been applied
+(ProviderType.GEMINI added to FALLBACK_ORDER). The test now passes normally.
+"""
+
+from __future__ import annotations
+
+import os
+from typing import Any
+from unittest.mock import MagicMock
+
+from behave import given, then, when  # type: ignore[import-untyped]
+
+from cleveragents.providers.registry import ProviderRegistry, ProviderType
+
+
+def _make_gemini_only_settings() -> Any:
+    """Return a fake Settings-like object with only gemini_api_key set."""
+    settings = MagicMock()
+    settings.openai_api_key = None
+    settings.anthropic_api_key = None
+    settings.google_api_key = None
+    settings.gemini_api_key = "sk-gemini-test-key"
+    settings.azure_api_key = None
+    settings.openrouter_api_key = None
+    settings.cohere_api_key = None
+    settings.groq_api_key = None
+    settings.together_api_key = None
+    settings.default_provider = None
+    settings.default_model = None
+    settings.azure_openai_endpoint = None
+    settings.azure_openai_api_version = None
+    settings.azure_openai_deployment = None
+    settings.openrouter_organization = None
+    return settings
+
+
+@given("a ProviderRegistry configured with only gemini_api_key set")
+def step_tdd_registry_gemini_only(context: Any) -> None:
+    context.tdd_registry = ProviderRegistry(settings=_make_gemini_only_settings())
+
+
+@given("CLEVERAGENTS_DEFAULT_PROVIDER is not set for tdd test")
+def step_tdd_unset_default_provider(context: Any) -> None:
+    context.original_provider_env = os.environ.get("CLEVERAGENTS_DEFAULT_PROVIDER")
+    os.environ.pop("CLEVERAGENTS_DEFAULT_PROVIDER", None)
+
+
+@when("I call get_default_provider_type on the tdd registry")
+def step_tdd_call_get_default_provider_type(context: Any) -> None:
+    context.tdd_result = context.tdd_registry.get_default_provider_type()
+
+
+@then("the tdd result should be ProviderType.GEMINI")
+def step_tdd_result_should_be_gemini(context: Any) -> None:
+    assert context.tdd_result == ProviderType.GEMINI, (
+        f"Expected ProviderType.GEMINI but got {context.tdd_result!r}. "
+        f"ProviderType.GEMINI should be in ProviderRegistry.FALLBACK_ORDER "
+        f"so that Gemini-only users get auto-selected provider."
+    )

features/tdd_gemini_fallback_order_4750.feature

@@ -0,0 +1,11 @@
+Feature: TDD: ProviderType.GEMINI missing from ProviderRegistry.FALLBACK_ORDER
+  As a user who configures only GEMINI_API_KEY
+  I want get_default_provider_type() to return ProviderType.GEMINI via the fallback chain
+  So that Gemini-only users can use auto-discovery without setting GOOGLE_API_KEY
+
+  @tdd_issue @tdd_issue_4750
+  Scenario: Gemini-only user gets GEMINI as default provider via fallback order
+    Given a ProviderRegistry configured with only gemini_api_key set
+    And CLEVERAGENTS_DEFAULT_PROVIDER is not set for tdd test
+    When I call get_default_provider_type on the tdd registry
+    Then the tdd result should be ProviderType.GEMINI

src/cleveragents/providers/registry.py

@@ -205,6 +205,7 @@ class ProviderRegistry:
         ProviderType.OPENAI,
         ProviderType.ANTHROPIC,
         ProviderType.GOOGLE,
+        ProviderType.GEMINI,
         ProviderType.AZURE,
         ProviderType.OPENROUTER,
         ProviderType.GROQ,