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/io/symbols.ts

@@ -35,3 +35,11 @@ export function bold(text: string): string {
 export function gray(text: string): string {
   return styleText('gray', text);
 }
+
+export function green(text: string): string {
+  return styleText('green', text);
+}
+
+export function red(text: string): string {
+  return styleText('red', text);
+}