fix(mirror): guard ISSUE_CLOSURE solved_by_pr write when issue was reopened

[andriypolanski]

Summary

• Guard ISSUE_CLOSURE writes with WHERE state = 'CLOSED' so a slow GraphQL fetch cannot repopulate solved_by_pr after a reopen webhook cleared it.
• Add unit tests for the happy path, reopen-during-fetch race, and early exit when the issue is already OPEN.

Related Issues

Fixes #199

Type of Change

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

Testing

• npm run build in packages/das passes
• npm run lint passes
• npm run test -- fetch.processor.spec.ts passes
• New tests cover:
  • solved_by_pr written when issue remains CLOSED after fetch
  • No row updated when issue was reopened during fetch (affected === 0)
  • Fetch skipped and solved_by_pr cleared when job starts with OPEN issue

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added