chore: move codex-exec unit tests into sibling files

[bolinfest]

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