Add ability to test database connection

Add the ability to test the connection the graph database.

The simplest version of this test would be to execute a database query using the connection settings. This would work across all supported database types (i.e. Neptune, Gremlin Server, SPARQL, etc).

* Test button within the connection dialog
* Run test query before schema sync to fail fast

## Potential solutions

* Execute small query
* New endpoint on the server
* On Neptune call status
* For generic connections call endpoint and ensure reachability
* Find any standard healthcheck API endpoints

Start by tracing the connection dialog and the schema sync flow, then compare how Neptune, Gremlin Server, SPARQL, and other supported database types expose reachability. The issue names no files or tests, so first identify those entry points and the existing connection handling. Done means a connection can be tested from the dialog and schema sync fails fast when the connection is invalid.

Written by the indexing model from the issue text.