feat(core): expose request body content streams

[sylvesterkaczmarek]

Summary

Add HttpRequestBody.content(): InputStream so custom HTTP client implementations can consume request bodies directly as streams.

Addresses the request-body portion of #664.

Problem

HttpResponse.body() already exposes an InputStream, but HttpRequestBody only exposes writeTo(OutputStream). Custom HttpClient adapters therefore have to bridge the push-style request body API themselves.

Changes

• add a JVM-default content(): InputStream method to HttpRequestBody
• keep existing third-party implementations compatible by providing a default fallback based on writeTo
• override the fallback for built-in JSON and multipart bodies
• stream multipart framing with SequenceInputStream, so InputStream-backed file parts are not buffered into memory
• add focused Kotlin regression coverage for fallback behavior and byte-for-byte multipart parity
• add a Java-facing regression proving existing implementations do not need to implement the new method

Compatibility

The repository compiles Kotlin interfaces with -Xjvm-default=all, so the new method is emitted as a Java default interface method. Existing Java and Kotlin implementations are not required to add an override.

The fallback implementation buffers only for third-party bodies that do not override content(). Built-in SDK request bodies override it, including streaming multipart bodies.

Validation

• branch is based directly on current upstream main at 715a30468d3232a5df4f6c6bec8b2175404eb8b9
• branch is 0 commits behind upstream
• diff is limited to HttpRequestBody, built-in request-body implementations, and focused tests
• focused regression tests are included for Kotlin and Java callers
• full repository validation is left to GitHub Actions because this environment does not have a complete local checkout/toolchain

Addresses #664

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[sylvesterkaczmarek]

I resolved this PR against current main while preserving the intended fix and newer upstream behavior. The branch is now mergeable and no longer has a base conflict. Could a maintainer please re-review it when convenient?

@codex review

[sylvesterkaczmarek]

Addressed the DEBUG logging streaming regression in f5997f3. LoggingHttpRequestBody now implements a streaming content() wrapper that logs incrementally instead of falling back to the buffering default, with a regression test whose delegate consumes content() and fails if writeTo() is used. The review thread is resolved. @codex review

[dpiet-oai]

Thank you, Sylvester, for the request-body streaming proposal, tests, and logging fix. We built on this work in #1245, which has now merged and adds HttpRequestBody.content(). Closing this PR as superseded by that implementation.