fix: gate issue bounty solver lookup on completed closures

[mkdev5]

Summary

Gate issue-bounty solver lookup on GitHub's completed closure reason.

check_github_issue_closed() previously treated any REST issue with state == "closed" as eligible for solver lookup. That allowed issues closed as not_planned, duplicate, or with missing/null state_reason to still return a merged PR solver, which issue_competitions() could turn into vote_solution() for an eligible miner.

This change requires closed issues to have state_reason == "completed" before find_solver_from_cross_references() runs. Non-completed or missing closure reasons now return a closed/no-solver result, preserving the existing response shape while preventing solution votes for non-completed closures.

Related Issues

Closes #979

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated

• Manually tested

• ./.venv/bin/pytest tests/utils/test_github_api_tools.py::TestCheckGithubIssueClosed -q

• ./.venv/bin/pytest tests/utils/test_github_api_tools.py -q

• ./.venv/bin/pytest tests/validator/test_issue_competitions_forward.py -q

• uv run pytest tests/ -q

• uv run ruff check

• uv run ruff format --check

• uv run pyright

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[anderdc]

Default-deny on non-completed closures matches the bounty-vote policy. Tests cover not_planned / duplicate / transferred / None / missing — solver_github_id=None routes those into the existing vote_cancel_issue path.