fix: handle empty error channel in returncode to prevent race condition

[kshiteej-mali]

/kind bug

The returncode property reads from the error channel after the WebSocket connection closes. On fast-exiting commands there is a race condition where the error channel message has not been buffered yet and read_channel() returns an empty string. The previous code passed this directly to yaml.safe_load() and accessed err['status'] without a null check, causing it to incorrectly return 0 even when the command failed with a non-zero exit code.

Fixes #2328 ( pod_exec() yields inconsistent results #2328 )

Reproducible by running command=['false'] in a loop 10 times - previously returned inconsistent exit codes (0 or 1 randomly).

Fixed a race condition in WSClient.returncode that caused pod exec commands with non-zero exit codes to intermittently return 0.

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