## Why
Passing extra descriptors to a PTY previously selected behavior intended for inherited escalation sockets: no EOF on stdin close, different signal exit statuses, and no default `SHELL`. Launch attachments need ordinary PTY behavior and must survive execution even when marked `FD_CLOEXEC`.
## What changed
- Add `ChildFds::Attached` alongside `ChildFds::Inherited` to distinguish launch attachments from legacy inherited descriptors.
- Preserve ordinary EOF, default `SHELL`, and signal exit status behavior for PTY attachments.
- Allow explicitly supplied close-on-exec descriptors through pipe and PTY launches without changing the parent's descriptor flags, using child-side flag updates and macOS spawn inheritance actions.
- Update existing callers to use `ChildFds::Inherited`.
## Testing
Add Unix coverage for attachment accessibility, unchanged parent descriptor flags, default `SHELL`, and signal exit statuses. Extend the PTY stdin-draining test to cover attachments and EOF delivery.
GitOrigin-RevId: 864aedc6c5717d8b43c9d71057c9f54409834509
jif
51d4562070