[EPAC-2300]: Isolate hansard index downloader tests

[riddim-developer-bot]

Summary

• Fixes EPAC-2300 by making the hansard search index downloader local path injectable while preserving /tmp/index.sqlite as the production default.
• Updates sqlitefile tests to use per-test temp index paths so schema mismatch validation cannot read stale shared /tmp/index.sqlite state.

Verification

• python3 scripts/intake/bugfix_spec.py validate .factory/intake/20260614-185342-epac-2300-backend-go-tests-failing-on-main/SPEC.md
• /usr/local/go/bin/go test -tags=integration ./... -count=1 (backend/hansard-search)
• /usr/local/go/bin/go test ./internal/adapter/sqlitefile -count=20 (backend/hansard-search)
• /usr/local/go/bin/go test ./... (backend/lobbying-index)
• /usr/local/go/bin/go test ./... (backend/bills-indexer)
• /usr/local/go/bin/go test ./... (backend/members-indexer)
• /usr/local/go/bin/go test ./... (backend/push-notification-dispatcher)
• python3 -m pytest scripts/ci/tests scripts/ci/test_backend_staging_smoke.py scripts/ci/test_ios_coverage_report.py

Release-Note: Omitted; CI/test-only backend change.