Cancel request while in progress

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

Requests that are building or loading can be cancelled in the TUI using Esc (they were always cancellable in the CLI just by ctrl-c).

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

Complexity in request state management means it's possible we get into an unexpected/invalid state. Mitigated with some debug asserts/logging. Also I made the logic as resilient as possible, so unexpected state transitions still get you to the more reasonable end state.

QA

How did you test this?

Added some unit tests, tested in the TUI.

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked