Preserve skill upload file paths

[kiwigitops]

Summary

• preserve directory paths in multipart filenames for skill and skill-version uploads
• keep the existing filename-stripping behavior for other multipart uploads
• add form-helper and skills.create regressions

Fixes #1807

Testing

• jest tests/form.test.ts --runInBand
• jest tests/api-resources/skills/skills.test.ts --runInBand -t "preserves directory paths"
• prettier --check src/internal/uploads.ts src/resources/skills/skills.ts src/resources/skills/versions/versions.ts tests/form.test.ts tests/api-resources/skills/skills.test.ts
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid preserving full URLs as upload filenames

When preserveFilePaths is enabled for skill uploads, getName still takes name from a Response.url, so files: [await fetch('https://cdn.example.com/my-skill/SKILL.md')] (allowed by Uploadable) now sends the multipart filename as the full URL instead of a relative path or basename. That gives the skill service an invalid tree entry and can leak the remote host in the uploaded path; URL-derived names should be normalized to a pathname/basename rather than returned verbatim.

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