fix(validator): filter stale closed mirror PRs

[JSONbored]

Summary

Defensively re-apply the repo lookback cutoff for CLOSED PRs returned by the mirror before they enter current credibility accounting.

What changed

• Pass the same per-repo since_by_repo cutoff map used for the mirror request into _maybe_add_pr.
• Drop returned CLOSED PRs whose created_at is older than that repo's cutoff.
• Add a regression test covering mixed stale/fresh CLOSED PRs from a mirror response.

Why

load_miner_prs() already computes per-repo lookback cutoffs and sends them to the mirror, but the loader currently assumes every returned PR is in-window. Since CLOSED PRs feed current credibility accounting, keeping the local cutoff preserves the existing invariant that out-of-window CLOSED PRs do not count as current attempts.

Proof

The new regression test builds a fake mirror response with:

• one stale CLOSED PR created 120 days ago
• one fresh CLOSED PR created 5 days ago
• a repo lookback of 30 days

Before this patch, the test fails because both CLOSED PRs are loaded:

assert [pr.pr.pr_number for pr in eval_.closed_prs] == [2]
E       assert [1, 2] == [2]

With this patch, the stale PR is filtered and only the fresh CLOSED PR remains in eval_.closed_prs.

I also checked the end-to-end loader/finalizer behavior locally with one current merged PR and one stale CLOSED PR returned by the mirror:

control:     closed_prs_loaded=0, credibility=1.0, eligible=true,  repo_score=100.0
before fix:  closed_prs_loaded=1, credibility=0.5, eligible=false, repo_score=0.0
after fix:   closed_prs_loaded=0, credibility=1.0, eligible=true,  repo_score=100.0

Validation

• uv run --extra dev pytest tests/validator/oss_contributions/mirror/test_load.py::TestLookbackWindow::test_stale_closed_pr_before_repo_lookback_is_dropped_defensively -q
• uv run --extra dev pytest tests/validator/oss_contributions/mirror tests/validator/test_blend_emission_pools.py tests/validator/test_scoring_classes.py tests/validator/test_scoring_resolver.py -q
• uv run --extra dev pytest -q
• uv run --extra dev ruff check gittensor/validator/oss_contributions/mirror/load.py tests/validator/oss_contributions/mirror/test_load.py
• git diff --check