fix: close pty writer in read_to_end to eliminate race condition

On Linux, writing to a PTY master after the child exits can succeed
(kernel buffers the data), unlike macOS which returns EIO. The
background thread sets writer=None after setting exit_status, but
read_to_end returns right after exit_status is set, creating a window
where write() still succeeds. Close the writer in read_to_end itself.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: branchseer

crates/pty_terminal/src/terminal.rs

@@ -194,6 +194,11 @@ impl Terminal {
         // Wait for exit status to be set by background thread
         let status = self.exit_status.wait().clone();
 
+        // Close the writer since the child has exited and all output has been consumed.
+        // This ensures subsequent write() calls fail immediately, rather than racing
+        // with the background thread which also closes the writer.
+        *self.writer.lock().unwrap() = None;
+
         Ok(status)
     }