fix: use indexed bracket notation for multipart array encoding

[rgarcia]

Summary

• The SDK's addFormValue uses key + '[]' for arrays, producing field names like files[][dest_path] and files[][file] instead of files[0][dest_path] and files[0][file].
• Without numeric indices, the server-side parser fails (strconv.Atoi("") on the empty string between []), returning 400 for all upload requests.
• Even if parsing succeeded, multiple files would be ambiguous — no way to pair each file with its dest_path.
• This is the same class of bug as kernel-go-sdk#86, where the Go SDK also dropped array indices.

Affected endpoints

Only two multipart endpoints have array-of-object fields and are affected:

• fs.upload — files: Array<{dest_path, file}>
• browsers.loadExtensions — extensions: Array<{name, zip_file}>

The other two multipart endpoints (deployments.create, extensions.upload) have no array fields so are unaffected by this change.

Fix

One-line change in src/internal/uploads.ts:

- await Promise.all(value.map((entry) => addFormValue(form, key + '[]', entry)));
+ await Promise.all(value.map((entry, i) => addFormValue(form, `${key}[${i}]`, entry)));

Failing tests added (commit 1)

Test	What it checks
fs.upload / single file should use indexed field names	Single-file upload field names contain [0]
fs.upload / multiple files should use distinct indexed field names	Multi-file upload produces [0], [1], [2] and content is correctly paired
fs.upload / field names should contain array indices (direct inspection)	No empty [] brackets, only numeric indices
browsers.loadExtensions / multiple extensions should use indexed field names	Same bug affects extensions upload

Repro

$ node repro.mjs
=== Test 1: single file upload ===
  FAILED: 400 400 invalid form field: files[][dest_path]

Test plan

• All 4 new tests fail on main (confirm bug exists)
• All 4 new tests pass after fix
• Full test suite passes (updated form.test.ts to expect bar[1] instead of bar[])
• Lint passes (./scripts/lint)