fix: defer PTY slave drop to prevent macOS data loss race

[branchseer]

Summary

Fixes a flaky is_terminal test failure on macOS (pty_terminal::tests::terminal::is_terminal) observed in CI run #22038626101.

Root Cause

Terminal::spawn previously dropped the parent's slave PTY fd immediately after spawning the child:

drop(pty_pair.slave); // Critical: drop slave so EOF is signaled when child exits

On macOS, when a child process writes output and exits quickly, all slave references close before the reader ever calls read() on the master. macOS then returns EIO on the master PTY without draining the output buffer, causing the child's output to be permanently lost. portable-pty converts EIO to Ok(0) (EOF), so read_to_end returns with zero bytes and screen_contents() is empty.

The failure is a race between child exit and the first read():

Scenario	Result
read() pending before child exits	Data delivered normally
read() called after all slave fds close	macOS returns EIO, data lost

Reproduction

Adding thread::sleep(Duration::from_secs(2)) before read_to_end in the is_terminal test deterministically reproduces the failure — the child always exits during the sleep, and the first read() returns 0 with empty screen contents.

Fix

Move the slave fd into the background monitoring thread instead of dropping it immediately in spawn(). The thread holds the slave alive until after child.wait() returns, then drops writer and slave in sequence:

thread::spawn(move || {
    child.wait();       // wait for exit
    writer = None;      // close writer
    drop(slave);        // NOW trigger EOF on the reader
});

This keeps the PTY in a "connected" state while the child runs, so the kernel output buffer is preserved regardless of when the reader starts.

Verification

• Stable repro: With a 2s sleep before read_to_end, the old code fails 100% of the time; the fix passes 100%.
• macOS: 30/30 runs of the full pty_terminal test suite pass.
• Windows (cargo xtest): 9/9 tests pass on aarch64-pc-windows-msvc.
• Full suite: cargo test (all crates) passes with no regressions.