fix: formData upload broken in debug builds

[riteshshukla04]

Summary:

So the formData Upload in fetch is broken on 0.85 on main.
I tracked down this PR #55456
Also reported here #56404
RCA:-

body.isOneShot()                                                                                                                                                          
body.writeTo(buffer)  // This is  draining inner file InputStreams   

Fix is to walk to each children of multipart and check for Oneshot

Changelog:

[ANDROID][FIXED] - FormData uploads broken in debug builds

Test Plan:

Tested on RN tester

[meta-codesync]

@alanleedev has imported this pull request. If you are a Meta employee, you can view this in D100258994.

[alanleedev]

@riteshshukla04 Thanks for the quick fix. We'll pick this into 0.85.1.

[react-native-bot]

This pull request was successfully merged by @riteshshukla04 in 0d1b70e

^{When will my fix make it into a release? | How to file a pick request?}

[riteshshukla04]

@riteshshukla04 Thanks for the quick fix. We'll pick this into 0.85.1.

Thanks for landing this. I will file a CP request

[react-native-bot]

This pull request was successfully merged by @riteshshukla04 in 9dfd26e

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[HarshitMadhav]

Hey — @riteshshukla04 @alanleedev thanks again for fixing and merging this; it fixed the FormData upload regression on Android.

Quick follow-up: by design we still return the literal "[Preview unavailable]" for :
(1) single one-shot bodies
(2) multipart bodies whose parts are one-shot.
That keeps things safe, but it also means the original DevTools pain from #55764 — no real payload preview for file/FormData uploads — is still there.

[HarshitMadhav]

I opened #56742 as a small follow-up on top of 0d1b70e. It keeps the same guarantee (we never call writeTo on a one-shot body),
but instead of those two placeholder returns it builds an actual preview: [Binary data, N bytes] for one-shot parts, plain text inlined where we can, all wrapped in a multipart envelope using the request’s real boundary so DevTools can parse it. NetworkEventUtilTest has coverage.

If @alanleedev @riteshshukla04 guys have bandwidth please review the PR, I’d love another pass since this directly stacks on top of your change.