test: demonstrate multipart array encoding bug with arrayFmt:"comma"

[rgarcia]

Summary

• apiform.MarshalRoot hardcodes arrayFmt:"comma" which drops array indices from multipart form field names. For endpoints with arrays of objects (like /fs/upload's files array), this produces ambiguous field names like files.dest_path / files.file instead of files.0.dest_path / files.0.file.
• Without indices, a server cannot pair file contents with their destination paths when multiple items are uploaded — every item gets identical field names.
• This is the root cause of kernel browsers fs upload returning 400 in production: the server-side parser requires an indexed format but gets index-less field names.

Failing tests added

Test	What it checks
TestUploadFilesMultipartEncoding/single_file	Even a single-file upload can't be parsed with standard indexed decoding
TestUploadFilesMultipartEncoding/multiple_files	Multi-file upload produces identical field names for all items
TestUploadFilesFieldNameFormat	Directly inspects field names, confirms they lack array indices
TestLoadExtensionsMultipartEncoding	Same bug affects the extensions upload endpoint

Repro output

Field names produced by SDK: [files.dest_path files.file files.dest_path files.file]

Two files uploaded, but all four field names are identical — no way to correlate which file goes with which dest_path.

Fix

The fix is to change MarshalRoot to use arrayFmt:"indices:dots" (or "indices:brackets") so it produces files.0.dest_path / files.0.file (or files[0].dest_path / files[0].file). Since this SDK is Stainless-generated, this likely needs a Stainless config change.

Test plan

• Tests fail on current main, confirming the bug exists
• After the arrayFmt fix, all four tests should pass

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Note

Medium Risk
Changes multipart form field naming for root-level arrays, which can affect server-side parsing expectations across upload-style endpoints. Risk is moderate because it alters request serialization but is covered by new integration-style tests.

Overview
Fixes multipart form encoding for root-level arrays by switching apiform.MarshalRoot from arrayFmt:"comma" to arrayFmt:"indices:dots", ensuring fields are emitted as indexed names (e.g., files.0.dest_path) instead of ambiguous repeated keys.

Adds a new multipart_array_encoding_test.go suite that spins up an httptest server to parse incoming multipart requests and asserts that Browsers.Fs.Upload and Browsers.LoadExtensions produce decodable, index-bearing field names for both single and multi-item arrays.

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