fix(bounties): use close events for solver attribution

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Summary

• Resolve issue-bounty solvers from GitHub's current ClosedEvent.closer instead of inferring attribution from PR cross-reference candidates.
• Require the close event to match the issue's current closedAt value and only attribute merged PR closers targeting the same repository.
• Keep PR cross-reference discovery available for issue-submission/PR-listing flows while removing it from bounty solver attribution.

What changed

• Added a GraphQL closure query for CLOSED_EVENT timeline entries and the associated PR closer fields.
• Updated bounty solver lookup to return a solver only from the authoritative current close event.
• Expanded GitHub API tests for non-PR closures, stale close events, missing closedAt, non-completed events, wrong-repo PR closers, and the check_github_issue_closed integration path.

Why

Cross-reference entries describe PR text references, not necessarily the PR that caused the issue's current closure. Bounty voting should follow GitHub's close-event source of truth so validators do not attribute a completed bounty to stale or unrelated closing references.

Validation

• uv run pytest tests/utils/test_github_api_tools.py tests/validator/test_issue_competitions_forward.py -q
• uv run ruff check gittensor/utils/github_api_tools.py tests/utils/test_github_api_tools.py
• uv run ruff format --check gittensor/utils/github_api_tools.py tests/utils/test_github_api_tools.py
• uv run pyright gittensor/utils/github_api_tools.py tests/utils/test_github_api_tools.py
• Verified a live closed upstream issue resolves through the close-event path with solver_lookup_failed=False.
• CodeRabbit CLI review completed and raised 0 issues.