fix(validator): cache issue-discovery state for zero-PR miners

[dripsmvcp]

• Always cache successful OSS-phase evaluations so issue-discovery-only miners get a cache anchor; previously the total_prs > 0 gate left them unrecoverable on transient mirror failures.
• Add a cached_eval.total_prs > 0 guard in the OSS-failure fallback so an issue-only entry can never masquerade as authoritative PR state.

Root Cause

Validator.store_or_use_cached_evaluation at neurons/validator.py:170-171 gated the cache write on miner_eval.total_prs > 0. Combined with MinerEvaluationCache.update_issue_discovery being a no-op when no entry exists (gittensor/classes.py:620-630), an issue-only miner (no PRs surviving load-time filtering) never got a cache entry. A later MirrorRequestError during issue discovery had nothing to restore, and the miner was scored zero — asymmetric with PR+issue miners. This disproportionately affects entrius/allways (issue_discovery_share == 1.0), where PR rewards are skipped entirely and issue discovery is the sole earning surface.

Fix

• Drop the total_prs > 0 gate at neurons/validator.py:170-171 — store() runs for every successful OSS fetch, including zero-PR miners.
• In the fallback branch, additionally check cached_eval.total_prs > 0 before swapping the cached eval in; an issue-only entry stays in the cache for update_issue_discovery / _restore_issue_discovery_from_cache but is never used as PR fallback. Uses the existing total_prs derived property rather than a new flag — the PR lists are already preserved in _build_cache_entry, so no new state to keep in sync.
• Update update_issue_discovery docstring to reflect the new invariant (entries now exist for zero-PR miners too).

Test Plan

• Regression tests added: tests/validator/test_store_or_use_cached_evaluation.py
  • TestZeroPrCaching::test_zero_pr_miner_is_cached_after_successful_oss_fetch
  • TestZeroPrCaching::test_update_issue_discovery_now_refreshes_zero_pr_entry
  • TestOssFallbackGuard::test_oss_failure_does_not_restore_issue_only_entry_as_pr_state
  • TestOssFallbackGuard::test_oss_failure_still_restores_entry_with_pr_data
• test_zero_pr_miner_is_cached_after_successful_oss_fetch fails on test without the validator change (verified by stashing the fix and re-running).
• All tests pass: pytest tests/validator/test_store_or_use_cached_evaluation.py tests/validator/issue_discovery/test_scan.py
• Output:

Fixes #1323