Add conformance testing for Explorer search backends

[marpaia]

Summary

This PR makes SynBioHub's Explorer integration explicitly configurable and testable against both SBOLExplorer and compatible search backends.

• Adds runtime-only environment overrides for enabling Explorer, selecting its endpoint, and controlling fallback behavior.
• Adds a strict Explorer mode that surfaces backend failures instead of silently retrying the query against the triplestore.
• Lets the test harness select the store and search backends independently:
  • Virtuoso with no external search backend
  • Virtuoso with SBOLExplorer
  • sbol-db with no external search backend
  • sbol-db with SBOLExplorer
  • sbol-db with its SBOLExplorer-compatible listener
• Adds an Explorer-specific integration test that explicitly requests an index update, waits for that operation to complete, and compares the complete /search/I0462 HTML against the established SynBioHub snapshot.
• Adds a pinned-corpus conformance harness for comparing submission, indexing, lifecycle, visibility, removal, and search behavior across Explorer implementations.
• Allows tests to use an explicit SBH_DOCKER_DIR, so the SynBioHub changes can be evaluated against an unmerged synbiohub-docker checkout.

Compatibility

Normal SynBioHub behavior is unchanged unless the new environment overrides are supplied. Explorer fallback remains enabled by default, while the test harness uses explicit backend selections so an unavailable Explorer cannot be mistaken for a passing result.

The Explorer-enabled test rows are opt-in and use the companion synbiohub-docker topology changes.

Testing

• The GitHub Integration testing workflow passes for this branch.

• The conformance runner and report-comparison unit tests pass:

  python3 -m unittest tests/search-backends/test_run_conformance.py tests/search-backends/test_compare_reports.py