fix: move inline codex-rs/core unit tests into sibling files (#14444)

## Why
PR #13783 moved the `codex.rs` unit tests into `codex_tests.rs`. This
applies the same extraction pattern across the rest of `codex-rs/core`
so the production modules stay focused on runtime code instead of large
inline test blocks.

Keeping the tests in sibling files also makes follow-up edits easier to
review because product changes no longer have to share a file with
hundreds or thousands of lines of test scaffolding.

## What changed
- replaced each inline `mod tests { ... }` in `codex-rs/core/src/**`
with a path-based module declaration
- moved each extracted unit test module into a sibling `*_tests.rs`
file, using `mod_tests.rs` for `mod.rs` modules
- preserved the existing `cfg(...)` guards and module-local structure so
the refactor remains structural rather than behavioral

## Testing
- `cargo test -p codex-core --lib` (`1653 passed; 0 failed; 5 ignored`)
- `just fix -p codex-core`
- `cargo fmt --check`
- `cargo shear`

codex-rs/core/src/user_shell_command.rs

@@ -55,61 +55,5 @@ pub fn user_shell_command_record_item(
 }
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::codex::make_session_and_context;
-    use crate::exec::StreamOutput;
-    use codex_protocol::models::ContentItem;
-    use pretty_assertions::assert_eq;
-
-    #[test]
-    fn detects_user_shell_command_text_variants() {
-        assert!(
-            USER_SHELL_COMMAND_FRAGMENT
-                .matches_text("<user_shell_command>\necho hi\n</user_shell_command>")
-        );
-        assert!(!USER_SHELL_COMMAND_FRAGMENT.matches_text("echo hi"));
-    }
-
-    #[tokio::test]
-    async fn formats_basic_record() {
-        let exec_output = ExecToolCallOutput {
-            exit_code: 0,
-            stdout: StreamOutput::new("hi".to_string()),
-            stderr: StreamOutput::new(String::new()),
-            aggregated_output: StreamOutput::new("hi".to_string()),
-            duration: Duration::from_secs(1),
-            timed_out: false,
-        };
-        let (_, turn_context) = make_session_and_context().await;
-        let item = user_shell_command_record_item("echo hi", &exec_output, &turn_context);
-        let ResponseItem::Message { content, .. } = item else {
-            panic!("expected message");
-        };
-        let [ContentItem::InputText { text }] = content.as_slice() else {
-            panic!("expected input text");
-        };
-        assert_eq!(
-            text,
-            "<user_shell_command>\n<command>\necho hi\n</command>\n<result>\nExit code: 0\nDuration: 1.0000 seconds\nOutput:\nhi\n</result>\n</user_shell_command>"
-        );
-    }
-
-    #[tokio::test]
-    async fn uses_aggregated_output_over_streams() {
-        let exec_output = ExecToolCallOutput {
-            exit_code: 42,
-            stdout: StreamOutput::new("stdout-only".to_string()),
-            stderr: StreamOutput::new("stderr-only".to_string()),
-            aggregated_output: StreamOutput::new("combined output wins".to_string()),
-            duration: Duration::from_millis(120),
-            timed_out: false,
-        };
-        let (_, turn_context) = make_session_and_context().await;
-        let record = format_user_shell_command_record("false", &exec_output, &turn_context);
-        assert_eq!(
-            record,
-            "<user_shell_command>\n<command>\nfalse\n</command>\n<result>\nExit code: 42\nDuration: 0.1200 seconds\nOutput:\ncombined output wins\n</result>\n</user_shell_command>"
-        );
-    }
-}
+#[path = "user_shell_command_tests.rs"]
+mod tests;