fix(api): regenerate raw_999_text in /api/acks/{id} detail response (SP3 P3 follow-up)

The P3 subagent shipped a Download button on /acks that fell back to
'no raw text' because the detail endpoint returned raw_json (the parsed
model dump) but never the regenerated X12.

Fix: detail endpoint now re-serializes raw_json via serialize_999 and
returns it as raw_999_text, so the operator can actually download the
999 file. List endpoint unchanged (keeps payload small).

backend/tests/test_api_999.py

@@ -108,3 +108,35 @@ def test_parse_999_rejected_fixture_surfaces_r_code(client: TestClient):
     assert body["ack"]["ack_code"] == "R"
     assert body["ack"]["rejected_count"] == 1
     assert body["ack"]["accepted_count"] == 0
+
+
+def test_get_ack_detail_regenerates_x12_text(client: TestClient) -> None:
+    """SP3 P3 follow-up: GET /api/acks/{id} must include `raw_999_text`
+    regenerated from raw_json so the operator can actually download the
+    999 file. The P3 subagent shipped `raw_999_text: None` as a stub."""
+    text = REJECTED.read_text()
+    post_resp = client.post(
+        "/api/parse-999",
+        files={"file": ("rejected_999.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert post_resp.status_code == 200
+    ack_id = post_resp.json()["ack"]["id"]
+    detail_resp = client.get(f"/api/acks/{ack_id}", headers={"Accept": "application/json"})
+    assert detail_resp.status_code == 200, detail_resp.text
+    body = detail_resp.json()
+    # raw_999_text must be present and non-empty X12.
+    assert "raw_999_text" in body
+    assert body["raw_999_text"] is not None
+    assert body["raw_999_text"].startswith("ISA*")
+    assert "AK1*" in body["raw_999_text"]
+    assert "AK9*" in body["raw_999_text"]
+    # IEA segment exists in output; serializer writes `IEA*1*<icn>~`.
+    assert "IEA*" in body["raw_999_text"]
+    assert body["raw_999_text"].rstrip().endswith("~")
+
+
+def test_get_ack_404_for_missing(client: TestClient) -> None:
+    """GET /api/acks/{id} returns 404 for a missing id (not 500)."""
+    resp = client.get("/api/acks/9999", headers={"Accept": "application/json"})
+    assert resp.status_code == 404