fix: count only canonical valid solved issues

[volcano303]

Summary

• Move valid_solved_count so it increments only after same-account and one-issue-per-PR canonical checks pass.
• Preserve solved credibility for non-canonical sibling issues while preventing them from satisfying the valid-solved eligibility gate.
• Add regression coverage for seven sibling issues closed by one PR, and update related canonical-rule fixtures.

Related Issues

Fixes #1269

Type of Change

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

Testing

• Tests added/updated
• Manually tested

Commands run:

.venv312/bin/python -m pytest tests/validator/issue_discovery/test_scan.py -q
.venv312/bin/python -m ruff check gittensor/validator/issue_discovery/scan.py tests/validator/issue_discovery/test_scan.py
.venv312/bin/python -m ruff format --check gittensor/validator/issue_discovery/scan.py tests/validator/issue_discovery/test_scan.py

Checklist

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