Add bounded batch connectivity checks for data sources

### Problem\n\nData source connectivity can only be tested one row at a time. Operators cannot validate all sources in the active list scope or review consolidated latency and failure results. Authenticated sources cannot safely be tested from persisted metadata because their query credentials are intentionally not stored.\n\n### Proposed solution\n\nAdd an on-demand batch connectivity drawer that loads every source matching the current filters, tests unauthenticated sources with a concurrency limit of three, explicitly skips authenticated sources without sending empty credentials, records latency and per-source messages, and exports the filtered result.\n\nThis is distinct from instance coverage analysis and reuses the existing test endpoint.

Start by locating the existing source connectivity test endpoint and the UI that owns the active-list filters. Trace how filtered sources are loaded, then verify the drawer handles bounded unauthenticated checks, skips authenticated sources without credentials, records latency and messages, and exports the filtered results.

Written by the indexing model from the issue text.