feat: Add comprehensive GPT-5 series model support

- Add GPT-5, GPT-5-mini, and GPT-5-nano models to unified configuration
- Implement proper thinking mode support via dynamic capability checking
- Add OpenAI provider model enumeration methods for registry integration
- Update tests to cover all GPT-5 models and their aliases
- Fix critical bug where thinking mode was hardcoded instead of using model capabilities

Breaking Changes:
- None (backward compatible)

New Models Available:
- gpt-5 (400K context, 128K output, reasoning support)
- gpt-5-mini (400K context, 128K output, efficient variant)
- gpt-5-nano (400K context, fastest/cheapest variant)

Aliases:
- gpt5, gpt5-mini, gpt5mini, gpt5-nano, gpt5nano, nano

All models support:
- Extended thinking mode (reasoning tokens)
- Vision capabilities
- JSON mode
- Function calling

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_openai_provider.py

@@ -253,14 +253,21 @@ class TestOpenAIProvider:
         assert call_kwargs["model"] == "o3-mini"  # Should be unchanged
 
     def test_supports_thinking_mode(self):
-        """Test thinking mode support."""
+        """Test thinking mode support based on model capabilities."""
         provider = OpenAIModelProvider("test-key")
 
-        # GPT-5 models support thinking mode (reasoning tokens)
+        # GPT-5 models support thinking mode (reasoning tokens) - all variants
         assert provider.supports_thinking_mode("gpt-5") is True
         assert provider.supports_thinking_mode("gpt-5-mini") is True
-        assert provider.supports_thinking_mode("gpt5") is True  # Test with alias
-        assert provider.supports_thinking_mode("gpt5mini") is True  # Test with alias
+        assert provider.supports_thinking_mode("gpt-5-nano") is True  # Now included
+
+        # Test GPT-5 aliases
+        assert provider.supports_thinking_mode("gpt5") is True
+        assert provider.supports_thinking_mode("gpt5-mini") is True
+        assert provider.supports_thinking_mode("gpt5mini") is True
+        assert provider.supports_thinking_mode("gpt5-nano") is True
+        assert provider.supports_thinking_mode("gpt5nano") is True
+        assert provider.supports_thinking_mode("nano") is True  # New alias for gpt-5-nano
 
         # O3/O4 models don't support thinking mode
         assert provider.supports_thinking_mode("o3") is False
@@ -270,6 +277,9 @@ class TestOpenAIProvider:
             provider.supports_thinking_mode("mini") is True
         )  # "mini" now resolves to gpt-5-mini which supports thinking
 
+        # Test invalid model name
+        assert provider.supports_thinking_mode("invalid-model") is False
+
     @patch("providers.openai_compatible.OpenAI")
     def test_o3_pro_routes_to_responses_endpoint(self, mock_openai_class):
         """Test that o3-pro model routes to the /v1/responses endpoint (mock test)."""