Improve SSE tests (#1546)

## Summary
- support fixture-based SSE data in tests
- add helpers to load SSE JSON fixtures
- add table-driven SSE unit tests
- let integration tests use fixture loading
- fix clippy errors from format! calls

## Testing
- `cargo clippy --tests`
- `cargo test --workspace --exclude codex-linux-sandbox`


------
https://chatgpt.com/codex/tasks/task_i_68717468c3e48321b51c9ecac6ba0f09

codex-rs/core/tests/previous_response_id.rs

@@ -11,6 +11,7 @@ mod test_support;
 use serde_json::Value;
 use tempfile::TempDir;
 use test_support::load_default_config_for_test;
+use test_support::load_sse_fixture_with_id;
 use tokio::time::timeout;
 use wiremock::Match;
 use wiremock::Mock;
@@ -42,12 +43,9 @@ impl Match for HasPrevId {
     }
 }
 
-/// Build minimal SSE stream with completed marker.
+/// Build minimal SSE stream with completed marker using the JSON fixture.
 fn sse_completed(id: &str) -> String {
-    format!(
-        "event: response.completed\n\
-data: {{\"type\":\"response.completed\",\"response\":{{\"id\":\"{id}\",\"output\":[]}}}}\n\n\n"
-    )
+    load_sse_fixture_with_id("tests/fixtures/completed_template.json", id)
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]