Summary

• Problem: Sequential reads of stdout then stderr can deadlock when a child process fills one pipe buffer (~64KB) while the reader is blocked on the other pipe
• Fix: Read both streams concurrently using asyncio.gather (Python) and Promise.all (Node.js)
• Tests: Added TestSimpleBoxConcurrentStreams and TestSyncSimpleBoxConcurrentStreams test classes

Test plan

• Verify Python async exec reads stdout/stderr concurrently via asyncio.gather
• Verify Python sync exec uses async internals with concurrent reads
• Verify Node.js exec reads stdout/stderr concurrently via Promise.all
• Run new concurrent stream test cases