Fix implemented:

• Extended app/api/schemas/openai.py
• Added full “tool-calls” data models (OpenAIToolCallFunctionModel, OpenAIToolCallModel).
• Enhanced ChatMessage with optional tool_calls and tool_call_id.
• Allowed unknown extra keys for forward-compatibility (extra="allow").
• Added a validator that explicitly permits content: null.
• No runtime logic needed to change – downstream provider adapters already ignore these extra fields and forward the payload unchanged.
• With these changes the exact same JSON you send to api.openai.com is now accepted by Forge