Validate error standardization fix for unsupported model handling

[Copilot]

Validates that commit b8dd547 correctly replaced inline error response construction with standardized GatewayError handling for unsupported model requests.

Changes Verified

• Error handling consistency: Unsupported model errors now use handleError(c, core.NewInvalidRequestError(...)) instead of manually constructing JSON responses
• API contract preserved: Response format and status codes remain identical (400 with invalid_request_error type)
• Test coverage: All existing tests pass, including TestChatCompletionUnsupportedModel

Before/After

// Before (cb82c33)
if !h.provider.Supports(req.Model) {
    return c.JSON(http.StatusBadRequest, map[string]interface{}{
        "error": map[string]interface{}{
            "type":    "invalid_request_error",
            "message": "unsupported model: " + req.Model,
        },
    })
}

// After (b8dd547)
if !h.provider.Supports(req.Model) {
    return handleError(c, core.NewInvalidRequestError("unsupported model: "+req.Model, nil))
}

Both paths produce identical responses but the latter ensures consistency with the centralized error handling system.

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