Add documentation for duplex property in streaming upload

[Copilot]

Addresses review feedback requesting explanation of the duplex property type assertion in streaming upload implementation.

Changes

• Added inline comment documenting:
  • Why duplex: 'half' is required for ReadableStream request bodies
  • What half-duplex communication means in this context
  • Why type assertion is needed (property not yet in standard TypeScript RequestInit)

// The 'duplex' property is required when using a ReadableStream as the request body.
// 'duplex: half' indicates half-duplex communication (one direction at a time),
// which is the mode needed for streaming request bodies with fetch().
// Type assertion is necessary because 'duplex' is not yet part of the standard
// TypeScript RequestInit type definition, though it's required by the fetch spec
// for streaming uploads.
const response = fetch(url, {
  method: 'PUT',
  body: webStream,
  duplex: 'half',
} as RequestInit & {duplex: 'half'})

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