Fix hanging on exec for missing command

[sjmiller609]

Note

Prevents hangs when a command is missing by sending binary error output and a JSON exitCode=127 over WebSocket, improving client close handling, and adding regression tests.

• Exec API (cmd/api/api/exec.go):
  • On exec error, write binary error output with \r\n and send JSON {"exitCode":127} via text message.
  • Clarify message types/comments; keep stdout binary streaming via wsReadWriter.
• CLI (cmd/exec/main.go):
  • In interactive mode, treat unexpected WebSocket closes as errors; normal closes default to exit code 0.
  • Non-interactive path: minor close-handling tidy-up; continues to parse {"exitCode":...} messages.
• Tests (lib/instances/exec_test.go):
  • Add regression tests ensuring non-existent commands fail quickly (TTY and non-TTY) without hanging and report "executable file not found".

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