fix: preserve binary request bodies over base64

[PradyumnaShome]

Summary

• encode ArrayBuffer and typed-array request bodies as base64 at the existing string wire boundary
• decode that base64 into the actual upload bytes on iOS and Android
• cover full buffers, typed-array subviews, empty bodies, non-ASCII bytes, and the worklet-safe fallback
• keep string, URLSearchParams, and form-data handling unchanged

Why

Binary request bodies are normalized in JavaScript today, but bodyBytes is then left unset. Even if it is provided, Android currently uploads a one-byte placeholder and iOS does not attach it to the request. This can drop or corrupt binary uploads.

This draft is an interim correctness fix: it intentionally retains the current bodyBytes?: string base64 transport and does not change the Nitro spec or generated ABI.

Tests

• bun --cwd packages/react-native-nitro-fetch test --runInBand src/__tests__/fetch.test.ts
• bun --cwd packages/react-native-nitro-fetch test --runInBand
• bun typecheck
• bun lint (0 errors; 2 pre-existing no-shadow warnings)
• git diff --check

Limitations / follow-up

The new coverage is a JavaScript contract test for the current base64 wire format. I did not run an iOS or Android app build in this checkout, so native decoding is not exercised by an integration test here.

#112 proposes the broader ArrayBuffer transport and generated binding changes. That remains the preferred follow-up; this draft is deliberately narrower so the request-body correctness fix can be reviewed independently without an ABI change.

[vercel]

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