Addresses #16781
Problem: `codex exec --ephemeral` backfilled empty `turn/completed`
items with `thread/read(includeTurns=true)`, which app-server rejects
for ephemeral threads.
This is a regression introduced in the recent conversion of "exec" to
use app server rather than call the core directly.
Solution: Skip turn-item backfill for ephemeral exec threads while
preserving the existing recovery path for non-ephemeral sessions.
Addresses #16560
Problem: `/status` stopped showing the source thread id in forked TUI
sessions after the app-server migration.
Solution: Carry fork source ids through app-server v2 thread data and
the TUI session adapter, and update TUI fixtures so `/status` matches
the old TUI behavior.
## Why
`codex-rs/exec/src/lib.rs` already keeps unit tests in a sibling
`lib_tests.rs` module so the implementation stays top-heavy and easier
to read. This applies that same layout to the rest of
`codex-rs/exec/src` so each production file keeps its entry points and
helpers ahead of test code.
## What
- Move inline unit tests out of `cli.rs`, `main.rs`,
`event_processor_with_human_output.rs`, and
`event_processor_with_jsonl_output.rs` into sibling `*_tests.rs` files.
- Keep test modules wired through `#[cfg(test)]` plus `#[path = "..."]
mod tests;`, matching the `lib.rs` pattern.
- Preserve the existing test coverage and assertions while making this a
source-layout-only refactor.
## Verification
- `cargo test -p codex-exec`
