…ttings

Bug bash feedback: switching the Explorer to S3 + Athena closes the Settings
modal almost immediately, with no clear signal that the destination is
configured correctly or that the backing resources exist. Adds a "Test
connection" button to the Data Source tab that runs read-only checks against
the current (unsaved) form values and reports a per-check pass/fail inline —
the modal stays open — so users can confirm a destination is reachable and
its config complete before saving or querying.

Host:
- New destinationTest.ts: testDestination(cfg) returns { ok, summary, checks[] }
  and dispatches per destination type. All probes are read-only.
    - S3 + Athena: required-field completeness; a note when neither workgroup
      nor result location is set (the "primary workgroup with no output
      location" trap); Glue table existence (reported as "will be created on
      Save" when absent, not a failure); and a real SELECT 1 that exercises the
      workgroup / output location / permissions.
    - DynamoDB: DescribeTable. Aurora: SELECT 1 via the RDS Data API.
      SQS: GetQueueAttributes. CloudWatch Logs: exact-name DescribeLogGroups.
- config.ts: factor the settings normalization out of readConfig into a shared
  helper and add configFromWireSettings(), so the test can validate the
  webview's unsaved values without persisting them.
- extension.ts: handle a new "testDestination" message; post the outcome back
  as a "destinationTestResult" payload (not the global error toast) so it
  renders inline in the modal.

Webview:
- SettingsModal.tsx: "Test connection" button plus a success/error Alert listing
  each check; the result clears when the modal reopens or the destination
  changes. App.tsx/types.ts: message plumbing and shared report types.

Tests: destinationTest.test.ts (per-destination success, connectivity failure,
and completeness paths) and config.test.ts (configFromWireSettings mapping,
defaults, and coercion). Host suite 156 passing; host + webview builds green.