fix: require completed closure for bounty solver lookup

[WT-WSL]

Summary

• Require GitHub issue state_reason to be completed before issue-bounty solver lookup runs.
• Default-deny non-completed, null, or missing closure reasons so those closures cannot produce vote_solution().
• Update GitHub API helper tests to use the session-backed request path and cover non-completed closure reasons.

Validation

• python3 -m py_compile gittensor/utils/github_api_tools.py tests/utils/test_github_api_tools.py tests/validator/test_issue_competitions_forward.py
• git diff --check
• Not run: pytest (pytest, uv, and pip are unavailable in this environment).

Close: #979

[anderdc]

Duplicate of #980 which is better scoped/implemented. Closing.