fix: gate MERGED solving PRs missing merged_at in mirror issue discovery

[Daedalus-Icarus]

Summary

A solving PR with state == 'MERGED' but merged_at is None is structurally unscoreable (time decay needs merged_at). _mirror_issue_for_scoring already drops it, but only after total_solved_issues and total_valid_solved_issues have been incremented. One corrupted record could push a miner past MIN_VALID_SOLVED_ISSUES, flipping eligibility and producing a non-zero discovery score from the other valid issues.

This PR adds the missing gate in _classify_issue so the corrupted record buckets as not-solved-closed before the counters bump — matching the defensive gate already present on the OSS mirror scoring path.

Changes

• gittensor/validator/issue_discovery/mirror_scan.py: reject MERGED solving PRs with merged_at is None in _classify_issue; document the gate in the module docstring.
• tests/validator/issue_discovery/test_mirror_scan.py: unit test on _classify_issue and a regression test verifying the counters stay at zero.

Close #919

Test plan

• pytest tests/validator/issue_discovery/test_mirror_scan.py
• Verify no behavior change for clean records (existing tests still green)

[xiao-xiao-mao]

DRIFT detected — original PR premise tackled paginating issue cross-reference timeline events in _PR_TIMELINE_QUERY, the current state may not align. Flagging for maintainer review.

• The body was rewritten from timeline pagination logic (GraphQL cursor loops, pageInfo handling) to a completely unrelated _classify_issue gate rejecting MERGED PRs with missing merged_at.
• Original body cited _PR_TIMELINE_QUERY, timelineItems, pageInfo, and Closes [BUG] Mirror issue discovery: MERGED solving PR with null merged_at inflates eligibility counters #919; current cites _classify_issue, _mirror_issue_for_scoring, and MIN_VALID_SOLVED_ISSUES, with zero overlap in concern.