refactor(upload): safer header handling and clearer chunk logic

[joshtrichards]

Some polishing of the upload helpers to avoid mutating passed-in headers and improve readability:

• normalize uploadData() options into explicit local variables / replace spread-defaults with explicit const locals in uploadData() for clarity and easier auditing
• clone headers before mutating in uploadData() and initChunkWorkspace()
• tidy getChunk() logic and variable naming
• improve inline and doc comments

Notes:

• I kept the blob creation behavior intact. It was tempting to remove the new Blob wrappers around file / file.slice() in getChunk(), but I did not want to risk a behavior change in case that normalization is relied on for browser compatibility or MIME-type handling.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI [sure; it suggested some comment tweaks]