Check property Solr before clearing its index

[alexandrutodor]

The all-ontology property indexing command clears the existing index before it knows whether the configured Solr core is reachable. A bad URL or unavailable core can therefore leave property search empty.

This checks the target core with a 10-second connection and read timeout before clearing. If the check fails, the command exits without clearing or indexing. The regular indexing client and its long-running timeouts are unchanged.

The test matrix also referenced the removed franzinc/agraph:v8.1.0 image. It now uses the available patch release v8.1.1, allowing the AllegroGraph unit tests to start.

The focused preflight test passes with 4 runs and 24 assertions.

[alexandrutodor]

The 4store test job passed. The AllegroGraph job stopped before running tests because franzinc/agraph:v8.1.0 no longer has a Docker manifest. That failure is unrelated to this change.

[alexandrutodor]

Updated the AllegroGraph test fixture to the available v8.1.1 patch image. Both unit-test jobs now pass: 31 tests, 670 assertions, 0 failures and 0 errors on both the 4store and AllegroGraph backends.