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>

providers/openai_provider.py

@@ -259,12 +259,10 @@ class OpenAIModelProvider(OpenAICompatibleProvider):
 
     def supports_thinking_mode(self, model_name: str) -> bool:
         """Check if the model supports extended thinking mode."""
-        # GPT-5 models support reasoning tokens (extended thinking)
-        resolved_name = self._resolve_model_name(model_name)
-        if resolved_name in ["gpt-5", "gpt-5-mini"]:
-            return True
-        # O3 models don't support extended thinking yet
-        return False
+        try:
+            return self.get_capabilities(model_name).supports_extended_thinking
+        except ValueError:
+            return False
 
     def get_preferred_model(self, category: "ToolModelCategory", allowed_models: list[str]) -> Optional[str]:
         """Get OpenAI's preferred model for a given category from allowed models.
@@ -303,3 +301,19 @@ class OpenAIModelProvider(OpenAICompatibleProvider):
             # Prefer balanced performance/cost models
             preferred = find_first(["gpt-5", "gpt-5-mini", "o4-mini", "o3-mini"])
             return preferred if preferred else allowed_models[0]
+
+    def get_model_configurations(self) -> dict[str, ModelCapabilities]:
+        """Get model configurations supported by this provider.
+
+        Returns:
+            Dict mapping model names to their ModelCapabilities
+        """
+        return self.SUPPORTED_MODELS.copy()
+
+    def get_all_model_aliases(self) -> dict[str, list[str]]:
+        """Get all model aliases supported by this provider.
+
+        Returns:
+            Dict mapping model names to their alias lists
+        """
+        return {model_name: caps.aliases for model_name, caps in self.SUPPORTED_MODELS.items()}