fix: preserve directory paths in multipart uploads

[daletyler1737]

Problem

The SDK's multipart form handling strips directory paths from filenames when uploading files. The getName function in internal/uploads.ts uses stripPath=true by default in createForm and multipartFormRequestOptions.

This breaks client.beta.skills.versions.create() which requires SKILL.md to be inside a top-level directory (e.g. my-skill/SKILL.md).

Solution

Change the default value of stripFilenames from true to false in both createForm and multipartFormRequestOptions.

Related Issue

Fixes #968