fix(bytesbuf_io): make ReadAsFuturesStream Send

[sandersaares]

[Copilot speaking]

ReadAsFuturesStream (returned by ReadExt::into_futures_stream()) was accidentally !Send. Its active_read field boxed the in-flight read future as a plain Pin<Box<dyn Future<...>>> with no + Send bound, and that single non-Send field made the whole stream !Send even when the underlying source is Send.

The future stored there comes from Read::read_any(), and the Read trait is declared with #[trait_variant::make(Send)] and documents that it "requires Send from both the implementation and any returned futures". So the future is always Send and the missing bound was purely an accidental omission.

This adds the + Send bound to the active_read field type and to the lifetime-extending transmute target types, plus a compile-time Send regression assertion (stream_is_send_when_source_is_send).

[Copilot]

Pull request overview

This PR fixes the bytesbuf_io ReadExt::into_futures_stream() adapter so the returned ReadAsFuturesStream is Send when the underlying Read source is Send, aligning the adapter with the Read trait’s documented “all returned futures are Send” contract.

Changes:

• Adds a missing + Send bound to the boxed in-flight read future (active_read), so it doesn’t force the whole stream to be !Send.
• Updates the mem::transmute target/source trait-object types to preserve the Send bound while extending the lifetime.
• Adds a compile-time regression test asserting the stream type is Send for a known-Send source (FakeRead).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (701cfc0) to head (8dfd901).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #592   +/-   ##
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  Coverage   100.0%   100.0%           
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  Files         359      359           
  Lines       27901    27901           
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  Hits        27901    27901           

Flag	Coverage Δ
linux	46.7% <100.0%> (-39.1%)	⬇️
linux-arm	46.7% <100.0%> (-40.1%)	⬇️
windows	46.7% <100.0%> (-40.5%)	⬇️

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