Fix (android): show payload preview for FormData and one-shot uploads in DevTools (#55764)

[HarshitMadhav]

Summary:

Follow-up to #56406. After that PR landed, NetworkEventUtil.getRequestBodyPreview still returns the literal string "[Preview unavailable]" in two cases:

The top-level RequestBody.isOneShot() is true (e.g. a single URI/file upload).
The body is a MultipartBody whose parts include any one-shot stream (the common case for FormData with files on Android).
Both branches exist for a good reason — calling writeTo() on a one-shot stream would drain it and break the real upload. This PR keeps that invariant but replaces the two return sites with structured previews:

Single one-shot body → [Binary data, N bytes] (or [Binary data] if contentLength() is unknown).
MultipartBody with one-shot parts → a multipart envelope using the request's actual boundary, with text parts inlined and one-shot parts replaced by [Binary data, N bytes]. The result parses cleanly against the on-the-wire Content-Type: multipart/form-data; boundary=..., so DevTools renders it the same way it renders any other multipart payload.
Net result: issue #55764 (no payload preview for FormData / file uploads in the React Native DevTools Network tab) is closed, with no change to what gets sent on the wire.

Changelog:

NetworkEventUtil.kt: replace the two "[Preview unavailable]" returns with binaryPartLabel(body) and a new previewMultipartWithBinaryParts(body) helper. No change to the existing all-text path.

NetworkEventUtilTest.kt: 6 new tests covering null body, plain string body, ProgressRequestBody unwrapping, multipart with text parts, multipart with a file part, and a single one-shot body.

[ANDROID] [FIXED] - Show request body preview for FormData and file uploads in DevTools Network tab

Test Plan:

1. ./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest --tests "com.facebook.react.modules.network.*" — 16/16 pass in NetworkEventUtilTest, full network suite green.

2. Manual: in RN-Tester debug build, POST a FormData with a file using fetch → DevTools Network → Payload tab now shows the multipart envelope with [Binary data, N bytes] instead of [Preview unavailable].

3. Manual: POST a single-file RequestBody from a URI → preview now shows [Binary data, N bytes].

Stream safety
The whole point of #56406's bail-outs was to avoid draining one-shot streams. This PR preserves that: it never calls writeTo() on a body whose isOneShot() is true. The only RequestBody methods invoked on one-shot bodies are contentType() and contentLength(), both of which are pure getters per the OkHttp docs.

Refs #55764, follows up on #56406.