fix(issue-discovery): clear stale per-repo issue fields on the scoring path

[kevin8600]

Summary

Issue discovery has two per-round reset paths that must clear a miner's per-repo issue fields identically, but they drifted:

• _clear_issue_discovery_fields (no-issues path) zeroes the issue fields on every repo_evaluation.
• _finalize_repo_issue_scores (scoring path) only rewrites the repos seen this round (sorted(set(repo_acc) | set(open_counts))), while _roll_up_issue_totals sums over all repo_evaluations.

So a repo scored in a prior round but absent this round keeps stale per-repo values that leak into the round-level totals.

Root cause. The only way a repo_evaluation carries prior-round issue data into a new round is the OSS cache-restore path: on github_pr_fetch_failed, store_or_use_cached_evaluation replaces the eval with the cached one (which carries repo_evaluations). Issue discovery is expected to recompute fresh fields on top (see the forward.py comment: "cached UIDs now have fresh issue-discovery fields"), but the scoring path never clears the repos it doesn't revisit, so _roll_up_issue_totals sums stale + fresh.

Impact. Inflated per-repo issue rows (persisted via bulk_store_evaluation) and round-level total_solved_issues / issue_discovery_score / is_issue_eligible, which also get re-written to the evaluation cache (update_issue_discovery) and compound across rounds. Consumed by CLI miner score and analytics. Current-round emission allocation reads the freshly-rebuilt issue_discovery_issues list, so this is a stored-data/reporting correctness bug, not an emissions change.

Fix. Extract the per-repo reset into a shared _reset_repo_issue_fields helper (removing the duplication that let the two paths drift) and call it at the top of _finalize_repo_issue_scores, mirroring _clear_issue_discovery_fields.

Related Issues

Fixes #1610

Type of Change

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

Testing

• Tests added/updated
• Manually tested

Added test_successful_scoring_clears_stale_issue_fields_for_repos_absent_this_round: pre-populates a repo_evaluation with prior-round issue data (simulating the OSS cache restore), returns mirror issues only for a different enabled repo, and asserts the stale repo and the round-level roll-up are both zeroed. The test fails on main (assert 7 == 0) and passes with the fix. Full issue-discovery suite green (73 passed); ruff, ruff format, and vulture clean.

Checklist

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