refactor: provider handling and core interfaces

[appleboy]

• Add error handling for invalid provider in commit.go
• Replace NewOpenAI initialization with a switch case for different providers
• Simplify the summary prefix retrieval logic in commit.go
• Create a new core/openai.go file defining Usage, Response, and Generative interface
• Create a new core/platform.go file defining Platform type and its methods
• Implement Completion and GetSummaryPrefix methods in openai.go for the core.Generative interface
• Refactor Completion method to completion in openai.go