fix(api-gateway): Handle array format for joins in /meta?extended endpoint

After PR #9800, joins are stored as arrays instead of objects in the
schema compiler. The transformJoins function in the API gateway was
still using Object.entries() which, when applied to arrays, returns
numeric indices as keys instead of actual join names.

This fix:
- Checks if joins is an array and preserves the name property
- Maintains backward compatibility with object format
- Adds test coverage for both array and object formats

Fixes the issue where join names appear as "0", "1", "2" instead of
their actual names in the /meta?extended endpoint response.

Author: igorlukanin

packages/cubejs-api-gateway/src/helpers/transformMetaExtended.ts

@@ -79,6 +79,16 @@ function transformJoins(joins: any) {
     return undefined;
   }
 
+  // Handle joins as array (new format after PR #9800)
+  if (Array.isArray(joins)) {
+    return joins.map((join: any) => ({
+      ...join,
+      name: join.name,
+      sql: stringifyMemberSql(join.sql),
+    }));
+  }
+
+  // Fallback for object format (legacy)
   return Object.entries(joins)?.map(([joinName, join]: [joinName: string, join: any]) => ({
     ...join,
     name: joinName,

packages/cubejs-api-gateway/test/helpers/transformMetaExtended.test.ts

@@ -219,6 +219,30 @@ describe('transformMetaExtended helpers', () => {
     expect(handledJoins?.length).toBe(2);
   });
 
+  test('transformJoins - array format', () => {
+    // Test new array format (after PR #9800)
+    const arrayJoins = [
+      {
+        name: 'PlaygroundUsers',
+        relationship: 'belongsTo',
+        sql: () => `{CUBE}.id = {PlaygroundUsers.anonymous}`,
+      },
+      {
+        name: 'IpEnrich',
+        relationship: 'belongsTo',
+        sql: () => `{CUBE.email} = {IpEnrich.email}`,
+      },
+    ];
+    
+    const handledJoins = transformJoins(arrayJoins);
+    expect(handledJoins).toBeDefined();
+    expect(handledJoins?.length).toBe(2);
+    expect(handledJoins?.[0].name).toBe('PlaygroundUsers');
+    expect(handledJoins?.[1].name).toBe('IpEnrich');
+    expect(handledJoins?.[0].sql).toBe('`{CUBE}.id = {PlaygroundUsers.anonymous}`');
+    expect(handledJoins?.[1].sql).toBe('`{CUBE.email} = {IpEnrich.email}`');
+  });
+
   test('transformPreAggregations', () => {
     expect(transformPreAggregations(undefined)).toBeUndefined();
     const handledPreAggregations = transformPreAggregations(MOCK_USERS_CUBE.preAggregations);