Handle cancelled OpenAI navigation without early success completion

[ratulsarna]

Supersedes #409.

Carries forward @spirosrap's fix to ignore NSURLErrorCancelled (-999) for OpenAI dashboard navigation, while avoiding early terminal success on cancellation.

• Ignore cancelled navigation callbacks without completing the continuation early.
• Keep non-cancelled failures as hard failures.
• Add tests that prove cancelled callbacks are non-terminal and later finish/failure drives completion.

Thanks @spirosrap for the original fix and test groundwork.