Make Gateway API compatible with OpenAI API

[Jeffwan]

Pull Request Description

I notice the error response format is completely incompatible. after checking https://github.com/openai/openai-openapi/tree/master?tab=readme-ov-file

• Gateway is missing required fields: type, param
• Gateway uses HTTP status code (int) for code instead of error code string
• 401 errors return invalid JSON: {"error":"unauthorized"}
• Error responses don't match OpenAI format. This affects: authentication errors, validation errors, rate limits, server errors, routing

Related Issues

Resolves: #1677, part of #846

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[gemini-code-assist]

Summary of Changes

Hello @Jeffwan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the Gateway API's error handling by making its error responses fully compatible with the OpenAI API specification. The changes involve introducing a structured approach to error generation, including new constants for error types and codes, and modifying existing functions to consistently produce detailed JSON error payloads that include 'type', 'code', and 'param' fields. This ensures better interoperability and a more predictable error experience for clients interacting with the gateway.

Highlights

• OpenAI Error Format Compatibility: The Gateway API's error responses are now aligned with the OpenAI API specification, including 'type', 'code', and 'param' fields for richer error details.
• Standardized Error Types and Codes: New constants for 'ErrorType' (e.g., 'invalid_request_error', 'authentication_error') and 'ErrorCode' (e.g., 'invalid_api_key', 'model_not_found') have been introduced to categorize errors consistently.
• Refactored Error Generation: Core error response functions ('generateErrorResponse', 'buildErrorResponse') have been updated to accept and utilize 'errorCode' and 'param' arguments, ensuring consistent error formatting across the gateway.
• Improved Error Messages: Specific error scenarios, such as unauthorized access (401), rate limits (429), and model not found, now return more precise and OpenAI-compliant messages and codes.

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[gemini-code-assist]

Code Review

This pull request makes important changes to align the gateway's error responses with the OpenAI API format, which is a great improvement for client compatibility. The approach of centralizing error generation logic in util.go is solid.

My review focuses on ensuring the new error format is fully compliant with OpenAI's standards. I've found a few areas where the error types and codes could be more accurate. I've also noted a minor issue with error handling in the JSON serialization logic. The suggested changes should help make the error handling more robust and fully compliant.

[Jeffwan]

this one has enough test coverage, I will merge it directly to unblock coming PRs.