Update tests to stop using sse_completed fixture (#10638)

Summary:
- replace the `sse_completed` fixture and related JSON template with
direct `responses::ev_completed` payload builders
- cascade the new SSE helpers through all affected core tests for
consistency and clarity
- remove legacy fixtures that were no longer needed once the helpers are
in place

Testing:
- Not run (not requested)

codex-rs/core/tests/fixtures/completed_template.json

@@ -1,16 +0,0 @@
-[
-  {
-    "type": "response.completed",
-    "response": {
-      "id": "__ID__",
-      "usage": {
-        "input_tokens": 0,
-        "input_tokens_details": null,
-        "output_tokens": 0,
-        "output_tokens_details": null,
-        "total_tokens": 0
-      },
-      "output": []
-    }
-  }
-]