fix: treat PTY hangup as EOF

[cmilesio]

What

• Treats the Unix PTY master EIO produced after the final slave closes as normal EOF.
• Applies that normalization only to PTY reads so real writer and output-forwarding failures still surface.
• Adds real-kernel PTY coverage for both successful commands and commands with a non-zero exit status.

Why

Execx v1.1.2 began preserving output-copy failures, but Linux uses EIO to signal a normal PTY hangup. That made completed PTY commands look like execution failures with read /dev/ptmx: input/output error.

The existing PTY tests used ordinary pipes, which end with EOF and could not reproduce the kernel behavior. This keeps the stricter output-error handling while correctly separating terminal shutdown from an actual I/O failure.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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