fix(backend-test-utils): prevent suite crash when no databases are available

When eachSupportedId() returned an empty array, describe.each([]) would
throw and crash the entire test suite. Return a placeholder ID instead,
so individual tests fail with a clear error rather than preventing the
entire suite from running.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@gmail.com>

packages/backend-test-utils/src/cache/TestCaches.ts

@@ -108,6 +108,9 @@ export class TestCaches {
   }
 
   eachSupportedId(): [TestCacheId][] {
+    if (this.supportedIds.length === 0) {
+      return [['MISSING' as TestCacheId]];
+    }
     return this.supportedIds.map(id => [id]);
   }
 

packages/backend-test-utils/src/database/TestDatabases.ts

@@ -124,6 +124,13 @@ export class TestDatabases {
   }
 
   eachSupportedId(): [TestDatabaseId][] {
+    if (this.supportedIds.length === 0) {
+      // Return a placeholder so that describe.each/it.each does not throw
+      // when no databases are available. The init() call will throw for
+      // the unknown ID, causing the test to be reported as failed rather
+      // than crashing the entire suite.
+      return [['MISSING' as TestDatabaseId]];
+    }
     return this.supportedIds.map(id => [id]);
   }