fix(issue bounties): avoid false cancel when closure timeline is truncated

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Summary

Closes #1411. Issue bounty validators must not vote cancel when GitHub’s closure timeline window omits the current close event. This aligns check_github_issue_closed() with issue_competitions/forward.py, which already skips cancel when solver_lookup_failed is true.

Changes

• Request up to 100 CLOSED_EVENT nodes in the closure GraphQL query.
• Treat “closed issue + completed events in window but no matching current close” as solver lookup failure (skip cancel in forward pass).
• Add regression test for truncated timeline behavior.

Real Behavior Proof

• Unit test test_truncated_closure_timeline_sets_solver_lookup_failed asserts solver_lookup_failed=True when stale close events fill the window but closedAt does not match any of them.

What I ran

python3 -m pytest tests/utils/test_github_api_tools.py::TestCheckGithubIssueClosed::test_truncated_closure_timeline_sets_solver_lookup_failed -q

What I observed

Test encodes the forward-pass guard: no false “no solver” path when the current close event is outside the fetched timeline window.