Handle discarded Arc tabs via pending URLs

[cgranier]

Summary

• add a helper that resolves the export URL for discarded tabs using pendingUrl when appropriate
• reuse the resolved URL when filtering tabs and building markdown context so Arc sleeping tabs are preserved
• add targeted unit tests covering discarded tab handling and restricted fallback behavior

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_69003d65ebb4832f898d127b0bc55aa7

[cgranier]

This did not work. I need to do more research into how Arc is sleeping and archiving tabs and see if there's a way to access them even in this state. Icing this for now.