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/client.rs

@@ -1261,101 +1261,5 @@ impl WebsocketTelemetry for ApiTelemetry {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::ModelClient;
-    use codex_otel::SessionTelemetry;
-    use codex_protocol::ThreadId;
-    use codex_protocol::openai_models::ModelInfo;
-    use codex_protocol::protocol::SessionSource;
-    use codex_protocol::protocol::SubAgentSource;
-    use pretty_assertions::assert_eq;
-    use serde_json::json;
-
-    fn test_model_client(session_source: SessionSource) -> ModelClient {
-        let provider = crate::model_provider_info::create_oss_provider_with_base_url(
-            "https://example.com/v1",
-            crate::model_provider_info::WireApi::Responses,
-        );
-        ModelClient::new(
-            None,
-            ThreadId::new(),
-            provider,
-            session_source,
-            None,
-            false,
-            false,
-            false,
-            None,
-        )
-    }
-
-    fn test_model_info() -> ModelInfo {
-        serde_json::from_value(json!({
-            "slug": "gpt-test",
-            "display_name": "gpt-test",
-            "description": "desc",
-            "default_reasoning_level": "medium",
-            "supported_reasoning_levels": [
-                {"effort": "medium", "description": "medium"}
-            ],
-            "shell_type": "shell_command",
-            "visibility": "list",
-            "supported_in_api": true,
-            "priority": 1,
-            "upgrade": null,
-            "base_instructions": "base instructions",
-            "model_messages": null,
-            "supports_reasoning_summaries": false,
-            "support_verbosity": false,
-            "default_verbosity": null,
-            "apply_patch_tool_type": null,
-            "truncation_policy": {"mode": "bytes", "limit": 10000},
-            "supports_parallel_tool_calls": false,
-            "supports_image_detail_original": false,
-            "context_window": 272000,
-            "auto_compact_token_limit": null,
-            "experimental_supported_tools": []
-        }))
-        .expect("deserialize test model info")
-    }
-
-    fn test_session_telemetry() -> SessionTelemetry {
-        SessionTelemetry::new(
-            ThreadId::new(),
-            "gpt-test",
-            "gpt-test",
-            None,
-            None,
-            None,
-            "test-originator".to_string(),
-            false,
-            "test-terminal".to_string(),
-            SessionSource::Cli,
-        )
-    }
-
-    #[test]
-    fn build_subagent_headers_sets_other_subagent_label() {
-        let client = test_model_client(SessionSource::SubAgent(SubAgentSource::Other(
-            "memory_consolidation".to_string(),
-        )));
-        let headers = client.build_subagent_headers();
-        let value = headers
-            .get("x-openai-subagent")
-            .and_then(|value| value.to_str().ok());
-        assert_eq!(value, Some("memory_consolidation"));
-    }
-
-    #[tokio::test]
-    async fn summarize_memories_returns_empty_for_empty_input() {
-        let client = test_model_client(SessionSource::Cli);
-        let model_info = test_model_info();
-        let session_telemetry = test_session_telemetry();
-
-        let output = client
-            .summarize_memories(Vec::new(), &model_info, None, &session_telemetry)
-            .await
-            .expect("empty summarize request should succeed");
-        assert_eq!(output.len(), 0);
-    }
-}
+#[path = "client_tests.rs"]
+mod tests;