fix: use indexed bracket notation for multipart array encoding

Changes array encoding in addFormValue from `key + '[]'` to
`key + '[' + i + ']'` so that arrays of objects produce indexed field
names (e.g. files[0][dest_path]) instead of ambiguous empty-bracket
names (e.g. files[][dest_path]).

Without indices, the server cannot parse the field names
(strconv.Atoi("") fails on the empty string between []) and cannot
pair file contents with destination paths for multi-file uploads.

Updates the existing form.test.ts assertion to expect the correct
indexed format (bar[1] instead of bar[]).

Made-with: Cursor

Author: rgarcia

src/internal/uploads.ts

@@ -174,7 +174,7 @@ const addFormValue = async (form: FormData, key: string, value: unknown): Promis
   } else if (isNamedBlob(value)) {
     form.append(key, value, getName(value));
   } else if (Array.isArray(value)) {
-    await Promise.all(value.map((entry) => addFormValue(form, key + '[]', entry)));
+    await Promise.all(value.map((entry, i) => addFormValue(form, `${key}[${i}]`, entry)));
   } else if (typeof value === 'object') {
     // Special case: env_vars should always be flattened for backward compatibility
     // with APIs that expect env_vars[KEY] format

tests/form.test.ts

@@ -122,6 +122,6 @@ describe('form data validation', () => {
       },
       fetch,
     );
-    expect(Array.from(form2.entries())).toEqual([['bar[]', 'foo']]);
+    expect(Array.from(form2.entries())).toEqual([['bar[1]', 'foo']]);
   });
 });