chore: move codex-exec unit tests into sibling files (#16581)

## 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`

codex-rs/exec/src/event_processor_with_jsonl_output.rs

@@ -621,59 +621,5 @@ impl EventProcessor for EventProcessorWithJsonOutput {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-    use pretty_assertions::assert_eq;
-    use tempfile::tempdir;
-
-    #[test]
-    fn failed_turn_does_not_overwrite_output_last_message_file() {
-        let tempdir = tempdir().expect("create tempdir");
-        let output_path = tempdir.path().join("last-message.txt");
-        std::fs::write(&output_path, "keep existing contents").expect("seed output file");
-
-        let mut processor = EventProcessorWithJsonOutput::new(Some(output_path.clone()));
-
-        let collected = processor.collect_thread_events(ServerNotification::ItemCompleted(
-            codex_app_server_protocol::ItemCompletedNotification {
-                item: ThreadItem::AgentMessage {
-                    id: "msg-1".to_string(),
-                    text: "partial answer".to_string(),
-                    phase: None,
-                    memory_citation: None,
-                },
-                thread_id: "thread-1".to_string(),
-                turn_id: "turn-1".to_string(),
-            },
-        ));
-
-        assert_eq!(collected.status, CodexStatus::Running);
-        assert_eq!(processor.final_message(), Some("partial answer"));
-
-        let status = processor.process_server_notification(ServerNotification::TurnCompleted(
-            codex_app_server_protocol::TurnCompletedNotification {
-                thread_id: "thread-1".to_string(),
-                turn: codex_app_server_protocol::Turn {
-                    id: "turn-1".to_string(),
-                    items: Vec::new(),
-                    status: TurnStatus::Failed,
-                    error: Some(codex_app_server_protocol::TurnError {
-                        message: "turn failed".to_string(),
-                        additional_details: None,
-                        codex_error_info: None,
-                    }),
-                },
-            },
-        ));
-
-        assert_eq!(status, CodexStatus::InitiateShutdown);
-        assert_eq!(processor.final_message(), None);
-
-        EventProcessor::print_final_output(&mut processor);
-
-        assert_eq!(
-            std::fs::read_to_string(&output_path).expect("read output file"),
-            "keep existing contents"
-        );
-    }
-}
+#[path = "event_processor_with_jsonl_output_tests.rs"]
+mod tests;