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/cli_tests.rs

@@ -0,0 +1,55 @@
+use super::*;
+use pretty_assertions::assert_eq;
+
+#[test]
+fn resume_parses_prompt_after_global_flags() {
+    const PROMPT: &str = "echo resume-with-global-flags-after-subcommand";
+    let cli = Cli::parse_from([
+        "codex-exec",
+        "resume",
+        "--last",
+        "--json",
+        "--model",
+        "gpt-5.2-codex",
+        "--dangerously-bypass-approvals-and-sandbox",
+        "--skip-git-repo-check",
+        "--ephemeral",
+        PROMPT,
+    ]);
+
+    assert!(cli.ephemeral);
+    let Some(Command::Resume(args)) = cli.command else {
+        panic!("expected resume command");
+    };
+    let effective_prompt = args.prompt.clone().or_else(|| {
+        if args.last {
+            args.session_id.clone()
+        } else {
+            None
+        }
+    });
+    assert_eq!(effective_prompt.as_deref(), Some(PROMPT));
+}
+
+#[test]
+fn resume_accepts_output_last_message_flag_after_subcommand() {
+    const PROMPT: &str = "echo resume-with-output-file";
+    let cli = Cli::parse_from([
+        "codex-exec",
+        "resume",
+        "session-123",
+        "-o",
+        "/tmp/resume-output.md",
+        PROMPT,
+    ]);
+
+    assert_eq!(
+        cli.last_message_file,
+        Some(PathBuf::from("/tmp/resume-output.md"))
+    );
+    let Some(Command::Resume(args)) = cli.command else {
+        panic!("expected resume command");
+    };
+    assert_eq!(args.session_id.as_deref(), Some("session-123"));
+    assert_eq!(args.prompt.as_deref(), Some(PROMPT));
+}