feat(providers): add OpenAI provider (`internal/providers/openai`)

[Nithwin]

Goal

Add native integration for OpenAI models (gpt-4o, o1, o3-mini) so users can switch providers smoothly.

Requirements

1. Create internal/providers/openai/provider.go implementing ai.Provider.
2. Translate ai.Message and ai.ToolDefinition structures into OpenAI schema formats (function tool calls).
3. Register via ai.Register("openai", New) inside init().