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/src/cyclone/api.py

@@ -971,6 +971,20 @@ def get_ack_endpoint(ack_id: int) -> dict:
         )
     body = _ack_to_ui(row)
     body["raw_json"] = row.raw_json
+    # Regenerate the X12 text from raw_json so the operator can download
+    # the actual 999 file. (SP3 P3 follow-up: list endpoint doesn't carry
+    # the regenerated text to keep payloads small; detail does.)
+    if row.raw_json:
+        try:
+            from cyclone.parsers.models_999 import ParseResult999
+            regenerated = ParseResult999.model_validate(row.raw_json)
+            icn = regenerated.envelope.control_number or "000000001"
+            body["raw_999_text"] = serialize_999(regenerated, interchange_control_number=icn)
+        except Exception as exc:  # noqa: BLE001 — never 500 on a regen failure
+            log.warning("Could not regenerate 999 for ack %s: %s", ack_id, exc)
+            body["raw_999_text"] = None
+    else:
+        body["raw_999_text"] = None
     return body