fix: upload body completion tracking

[HenriqueSFernandes]

CountingBody relied on size_hint() to detect the final upload frame, but UploadBody does not provide a bounded size hint.

Use the wrapped body’s is_end_stream() state and guard final events so completion is reported exactly once.

Closes #42

[Copilot]

Pull request overview

This PR fixes upload completion tracking in CountingBody by replacing a size_hint()-based “last frame” heuristic (which fails for bodies without bounded size hints, like UploadBody) with is_end_stream() checks on the wrapped body, ensuring completion events are emitted exactly once.

Changes:

• Switch “last frame” detection from size_hint() to the wrapped body’s is_end_stream() state.
• Ensure final ByteCount + Finished events are guarded by sent_finished in both the last-frame path and the Poll::Ready(None) path.

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