Validate error standardization fix in ChatCompletion handler

[Copilot]

Validated that the inline error construction in ChatCompletion handler has been replaced with standardized error handling using core.NewInvalidRequestError() and handleError().

Changes validated:

• Bind errors now use core.NewInvalidRequestError() instead of hardcoded error type and manual JSON construction
• Unsupported model errors follow the same pattern
• Both error cases delegate to handleError() helper for consistent response formatting

Before (commit cb82c33):

if err := c.Bind(&req); err != nil {
    return c.JSON(http.StatusBadRequest, map[string]interface{}{
        "error": map[string]interface{}{
            "type":    "invalid_request_error",
            "message": "invalid request body: " + err.Error(),
        },
    })
}

After (commit b8dd547):

if err := c.Bind(&req); err != nil {
    return handleError(c, core.NewInvalidRequestError("invalid request body: "+err.Error(), err))
}

All handler tests pass. Error responses maintain consistent JSON structure via GatewayError.ToJSON().

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.