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/fixtures/incomplete_sse.json

@@ -0,0 +1,3 @@
+[
+  {"type": "response.output_item.done"}
+]