fix: count old open mirror issues for issue spam multiplier

[MkDev11]

Summary

Splits mirror issue-discovery scoring from open-issue spam counting.

The scoring path still uses the lookback-bounded mirror /issues response, but total_open_issues is now resolved independently so older still-open issues count toward calculate_open_issue_spam_multiplier().

This also adds support for a future mirror open_issue_count response field. Until the mirror serves that field, the validator falls back to a separate all-time current-open issue fetch and filters it to mirror-enabled repos.

Related Issues

Fixes #929

Type of Change

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

Testing

• Tests added/updated
• Manually tested

Commands run:

uv run pytest tests/validator/issue_discovery/test_mirror_scan.py tests/utils/test_mirror_models.py tests/utils/test_mirror_client.py
uv run ruff check gittensor/utils/mirror/models.py gittensor/validator/issue_discovery/mirror_scan.py tests/validator/issue_discovery/test_mirror_scan.py tests/utils/test_mirror_models.py
uv run ruff format --check gittensor/utils/mirror/models.py gittensor/validator/issue_discovery/mirror_scan.py tests/validator/issue_discovery/test_mirror_scan.py tests/utils/test_mirror_models.py
uv run pyright
git diff --check

Checklist

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

[anderdc]

Still debating on merging this — putting a pin in it.

[MkDev11]

thanks @anderdc , totally understand you.

My only push for merging is that this is the exact follow-up called out in #796: scoring should stay lookback-bounded, but the open-issue spam count should be current open issue load.

The PR keeps the scoring fetch unchanged. It only separates total_open_issues, prefers the future open_issue_count field when present, and falls back to current behavior if the extra count fetch fails.

If the concern is the temporary second mirror request, I can drop that fallback and only wire the future field, but that would leave #929 unfixed until the mirror serves it.