[BUG] Mirror issue discovery: MERGED solving PR with null `merged_at` inflates eligibility counters

[statxc]

Description

In _classify_issue (gittensor/validator/issue_discovery/mirror_scan.py), a solving PR reported with state == 'MERGED' but merged_at is None is not gated. The record reaches _mirror_issue_for_scoring, which correctly drops it as unscoreable — but only after total_solved_issues and total_valid_solved_issues have already been incremented.

A single corrupted record can therefore push a miner from valid_solved_count == MIN_VALID_SOLVED_ISSUES - 1 (ineligible) to MIN_VALID_SOLVED_ISSUES (eligible), flipping is_issue_eligible and producing a non-zero issue_discovery_score from the other valid records that would otherwise be discarded.

The OSS mirror scoring path already rejects this exact shape with a "MERGED but missing merged_at" guard; the issue-discovery path was missing the equivalent gate.

Steps to Reproduce

1. Set up a miner with exactly MIN_VALID_SOLVED_ISSUES - 1 clean solved mirror issues (one short of the eligibility floor).
2. Add one mirror issue whose solving_pr.state == 'MERGED' and solving_pr.merged_at is None.
3. Run run_mirror_issue_discovery against this miner.

Expected Behavior

The corrupted record is bucketed as not-solved-closed. total_valid_solved_issues stays at MIN_VALID_SOLVED_ISSUES - 1, is_issue_eligible is False, and issue_discovery_score is 0.

Actual Behavior

_classify_issue returns 'solved'. total_solved_issues and total_valid_solved_issues are incremented to MIN_VALID_SOLVED_ISSUES. _mirror_issue_for_scoring then drops the corrupted record, but the eligibility flag has already flipped to True, and the miner receives a non-zero issue_discovery_score derived from the other valid records.

Environment

• OS: Linux (Ubuntu 24.04)
• Python version: 3.11
• Commit/Version: test branch HEAD (pre-fix)

Additional Context

• Affected file: gittensor/validator/issue_discovery/mirror_scan.py — _classify_issue.
• Reference for the equivalent OSS-side gate: gittensor/validator/oss_contributions/mirror/scoring.py ("MERGED but missing merged_at" rejection).
• Fix: add a sp.merged_at is None → 'not-solved-closed' branch in _classify_issue after the state != 'MERGED' check, so the gate fires before the counters increment. See PR fix: gate MERGED solving PRs missing merged_at in mirror issue discovery #920.

[xiao-xiao-mao]

MINOR DRIFT detected — original premise tackled missing PRs due to unpaginated 50-event cap in timeline query, the new state may not fully align. Flagging for maintainer review.

• The title was changed from a pagination bug in _PR_TIMELINE_QUERY to a different bug about null merged_at inflating eligibility counters, while the body still describes the original pagination issue.
• The new title describes a fundamentally different problem than the body content, creating a mismatch that suggests the issue may have been repurposed.

[xiao-xiao-mao]

DRIFT detected — original premise tackled GraphQL pagination gap in _PR_TIMELINE_QUERY for cross-reference events, no longer aligns with the new issue.

• The original reported that only the first 50 cross-reference timeline events were read, causing submissions and solver lookup to miss PRs; the current issue describes an entirely different bug in mirror issue discovery classification.
• Original was a pagination gap in github_api_tools.py; current is a null merged_at guard in mirror_scan.py.

If you believe the issue is valid, please open a fresh (non-duplicate) issue.

[xiao-xiao-mao]

DRIFT detected — original premise tackled missing pagination in _PR_TIMELINE_QUERY for cross-reference events, no longer aligns with the new issue.

• The current body describes an entirely different bug (null merged_at guard in mirror issue discovery classification), replacing the original pagination gap report wholesale.
• Original was about GraphQL timeline event pagination in github_api_tools.py; current is about a missing null check in mirror_scan.py _classify_issue.

If you believe the issue is valid, please open a fresh (non-duplicate) issue.