test: migrate FilesList header specs to testing Pinia

[ColumbusLabs]

Resolves: #7940

📝 Summary

• migrate the FilesList table header component tests from real Pinia instances to createTestingPinia()
• provide isolated store state through initialState for sorting, file, and selection scenarios
• keep sorting actions stubbed when tests only verify calls
• preserve real file and selection action behavior where rendering or state transitions are under test

🧪 How to test

npm run ts:check
npx vitest run \
  src/tests/views/FilesList/FilesListTableHeaderButton.spec.ts \
  src/tests/views/FilesList/FilesListTableHeader.spec.ts
npm run lint
npm test

Local results with Node 24.18.0:

• 31/31 affected tests passed
• 3,040/3,040 frontend tests passed
• TypeScript check passed
• ESLint passed

✅ Checklist

• I have read and followed the contribution guide.
• The change is limited to the two test suites named in the issue.
• Existing test intent and application behavior are preserved.
• The commit includes the required DCO sign-off.

🤖 AI (if applicable)

• The content of this PR was partially or fully generated using AI

OpenAI Codex assisted with repository inspection, the test migration, and verification. I reviewed the changes and take responsibility for the submission.