fix: sync pr_state for stale-closed mirror PRs excluded from scoring

[Dexterity104]

Summary

Stale CLOSED PRs from the mirror path now land in a storage-only bucket so the pull_requests row's pr_state flips from OPEN to CLOSED on the next evaluation. Before this change, the loader returned early without recording them, leaving phantom OPEN rows on dashboards for every mirror_enabled: true repo.

The fix mirrors PR #769, which solved the same problem on the legacy GraphQL path. The storage layer was already wired (stale_closed_pull_requests flows through store_pull_requests_bulk, and pr_state is in the DO UPDATE SET clause of BULK_UPSERT_PULL_REQUESTS), so the work is to add the matching mirror bucket and route it into the existing call.

Changes:

• MirrorMinerEvaluation gains a stale_closed_prs field
• _maybe_add_pr appends pre-lookback CLOSED PRs there instead of returning silently
• MinerEvaluation gains a parallel mirror_stale_closed_prs slot
• combine propagates the bucket onto the legacy evaluation
• DatabaseStorage.store_evaluation concatenates the adapted mirror list onto the existing stale storage call (the four parallel store_pull_requests_bulk calls also get folded into a single loop over a pr_buckets table for clarity)
• The mirror summary log now reports stale-closed count for observability

These PRs do not enter scoring or credibility math, so totals (total_closed_prs, merged_count for check_eligibility, etc.) stay correct.

Related Issues

Fixes #1031

Type of Change

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

Testing

• Tests added/updated
• Manually tested

Test coverage:

• tests/validator/oss_contributions/mirror/test_load.py: the existing stale-closed test now also asserts the PR lands in stale_closed_prs and that total_closed_prs reflects only the scoring bucket
• tests/validator/oss_contributions/mirror/test_combine.py: setup now seeds a stale entry, and test_aggregate_pr_count_properties_sum_both_paths proves it does not inflate total_closed_prs
• tests/validator/utils/test_storage_mirror.py: new test_stale_closed_lists_concatenated covers legacy plus mirror concatenation, PullRequest adaptation, and identity propagation through the call at call_args_list[3]

Full suite: 602 passed, ruff clean.

Checklist

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

[anderdc]

The validator's pull_requests table is a scoring artifact (who got credit and when), not the source of truth for current PR state. For mirror-enabled repos, das-github-mirror tracks live state continuously and is the canonical source — downstream consumers wanting current open/closed state should read from the mirror, not the validator table. Phantom OPEN rows are a non-issue if you don't treat the validator table as state-of-the-world. Closing.