fix: set duplex for streaming uploads

[KnightNiwrem]

Note

This PR was primarily written by GPT-5.6 Sol xhigh in ChatGPT Work.

Summary

Add duplex: "half" to the request configuration produced for streaming multipart uploads.

The value is kept as a string literal with as const, and JSON-only API calls remain unchanged.

Why

The v2 branch changed from node-fetch to Node's native, Undici-backed fetch. That change did not automatically solve the streaming request requirement: Node 25 still rejects grammY's multipart ReadableStream body unless duplex: "half" is supplied.

Draft PR #949 reproduces the failure through a normal Api instance and api.sendDocument. The corresponding fix on main was made in #913.

Closes #949

Verification

• deno fmt --check src/payload.ts
• deno lint src/payload.ts
• type-checked the resulting request configuration through src/client.ts
• confirmed on Node 25.9.0 that Api.sendDocument completes and reaches a local HTTP endpoint exactly once without a caller-provided baseFetchConfig.duplex