Copilot assignment polling continues waiting after request cancellation #3703

Problem

Both assign_copilot_to_issue and the direct-assignment path of assign_copilot_to_issue_with_intent update the issue successfully, then optionally poll for a linked pull request. Cancellation during that polling does not interrupt time.Sleep, and canceled query errors are ignored. The handler can wait through the rest of the default eight-second polling window after the caller has canceled.

Reproduction and expected behavior

Using the real tool handlers with a local GraphQL HTTP mock, complete the issue assignment successfully, return no linked PR from the first poll, and cancel the request during the inter-attempt delay. With a one-hour PollConfig delay under testing/synctest, both handlers take the full virtual hour instead of returning at cancellation (tested at zero and one second).

Expected: stop optional polling promptly and preserve the already-confirmed successful assignment with the existing pending-PR result. Cancellation should not retry or claim to undo the completed assignment. Uncancelled calls should keep their existing delay, attempt count and result format.

Environment and scope

• main at 99d6d65, Go 1.27.3, macOS arm64.
• Entire reproduction uses local HTTP mocks and a virtual clock; no live GitHub mutation.
• Related fix: make Copilot assignment session state truthful #3307 changes session detection reporting but retains the unconditional sleeps.
• A local fix and regression tests are prepared.