fix: Surface billing-related errors to the user

[dustinbyrne]

Problem:
Currently, when a user exceeds their monthly request limit for a service like Copilot, the application does not provide any specific feedback. This leads to a confusing user experience where the service silently stops working, leaving the request hanging and the user unsure of the cause.

Solution:
This change introduces proper handling for billing-related errors to ensure users receive clear and actionable feedback.

1. A new, specific PaymentRequiredError has been created to represent this state. This error contains a static, user-friendly message explaining that the service requires payment or a subscription check.

2. The OpenAI completion service has been updated to detect HTTP 402 status codes in API responses.

3. When a 402 error is detected, it is wrapped in the new PaymentRequiredError and thrown. The frontend will present the message of this error to the user.

Impact:
Users who have exceeded their usage limits will now see a clear error message explaining the issue, allowing them to take corrective action (e.g., checking their subscription or billing status). This significantly improves transparency and the overall user experience by turning a silent failure into actionable feedback.

Testing:
Unit tests have been added to verify that:

• An API error with a 402 status code correctly results in a PaymentRequiredError.
• The new error is propagated correctly through the completion service and is not retried.

[Copilot]

Pull Request Overview

This PR introduces proper handling for billing-related errors by creating a new PaymentRequiredError class and integrating it with the OpenAI completion service to provide clear user feedback when payment or subscription issues occur.

Key changes:

• Added a new PaymentRequiredError exception class with a user-friendly message
• Updated OpenAI completion service to detect HTTP 402 status codes and throw the new error
• Added comprehensive test coverage for the new error handling behavior

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/navie/src/services/completion-service.ts	Defines the new PaymentRequiredError class and adds error handling logic
packages/navie/src/services/openai-completion-service.ts	Implements HTTP 402 status code detection and error throwing
packages/navie/test/services/completion-service.spec.ts	Adds unit tests for the base completion service error handling
packages/navie/test/services/openai-completion-service.spec.ts	Adds unit tests for OpenAI-specific 402 error handling

[dustinbyrne]

It's worth mentioning that I think our error handling overall looks like it's in need of some improvements. Other 4xx errors may be causing the same sort of erroneous behavior (hanging requests, no user feedback). I think that may be a good follow on item.