SDK 0.4.27: full duplex exec I/O

Exec now sends stdin and drains stdout/stderr concurrently in both sync and async clients. This avoids deadlocks with bounded server output backpressure. Failure and cancellation clean up peer I/O and stop the exec while preserving the original error.

Upgrade clients using large stdin before deploying server 0.5.114rc24. Streaming handle users must also drain events while writing input.

Validation: focused SDK client/duplex tests and real Linux HTTP/process tests; wheel and source distribution built from dc62af7.