feat(cli): extend ktx connection test to every supported driver (#92)

* feat(cli): extend `ktx connection test` to every supported driver

Dispatch by driver: native DBs now call `connector.testConnection()`
(was `introspect(dryRun)`), looker/notion/metabase hit their auth
endpoints, and dbt/metricflow/lookml run `git ls-remote` via the
existing `testRepoConnection` helper. Unknown drivers exit 1 with a
listing of supported ones.

* feat(cli): add `ktx connection test --all` summary list

Tests every configured connection in parallel and renders a single
Clack-style list (◇/│/◆/└, green ✓ / red ✗) consistent with sl list,
with per-row detail and a passed/failed footer. Exits non-zero if any
connection fails. Single-id `ktx connection test` output is preserved.

* fix(cli): read metabase status url from api_url

`ktx status` was probing `url` / `base_url` on metabase connections, but
ktx.yaml stores it as `api_url`, so the field always reported "url not
set". Read `api_url` directly and align the warning text with the actual
key.

packages/cli/src/print-command-tree.test.ts

@@ -16,7 +16,7 @@ describe('renderKtxCommandTree', () => {
       expect(topLevel).toContain(expected);
     }
 
-    expect(output).toContain('│   └── test <connectionId>');
+    expect(output).toContain('│   └── test [connectionId]');
     expect(output).not.toContain('│   ├── add');
     expect(output).not.toContain('│   ├── remove');
     expect(output).not.toContain('│   ├── map');