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/commit_attribution.rs

@@ -29,48 +29,5 @@ fn resolve_attribution_value(config_attribution: Option<&str>) -> Option<String>
 }
 
 #[cfg(test)]
-mod tests {
-    use super::build_commit_message_trailer;
-    use super::commit_message_trailer_instruction;
-    use super::resolve_attribution_value;
-
-    #[test]
-    fn blank_attribution_disables_trailer_prompt() {
-        assert_eq!(build_commit_message_trailer(Some("")), None);
-        assert_eq!(commit_message_trailer_instruction(Some("   ")), None);
-    }
-
-    #[test]
-    fn default_attribution_uses_codex_trailer() {
-        assert_eq!(
-            build_commit_message_trailer(None).as_deref(),
-            Some("Co-authored-by: Codex <noreply@openai.com>")
-        );
-    }
-
-    #[test]
-    fn resolve_value_handles_default_custom_and_blank() {
-        assert_eq!(
-            resolve_attribution_value(None),
-            Some("Codex <noreply@openai.com>".to_string())
-        );
-        assert_eq!(
-            resolve_attribution_value(Some("MyAgent <me@example.com>")),
-            Some("MyAgent <me@example.com>".to_string())
-        );
-        assert_eq!(
-            resolve_attribution_value(Some("MyAgent")),
-            Some("MyAgent".to_string())
-        );
-        assert_eq!(resolve_attribution_value(Some("   ")), None);
-    }
-
-    #[test]
-    fn instruction_mentions_trailer_and_omits_generated_with() {
-        let instruction = commit_message_trailer_instruction(Some("AgentX <agent@example.com>"))
-            .expect("instruction expected");
-        assert!(instruction.contains("Co-authored-by: AgentX <agent@example.com>"));
-        assert!(instruction.contains("exactly once"));
-        assert!(!instruction.contains("Generated-with"));
-    }
-}
+#[path = "commit_attribution_tests.rs"]
+mod tests;