The 999, 277CA, and TA1 parsers already enforce envelope correctness at
the parser level (parse_999.py line 290 raises 'No AK9 segment found';
parse_277ca.py line 298 raises 'Expected ST*277 or ST*277CA'; parse_ta1.py
line 111 raises 'Expected TA1, got <other>'). These tests lock the HTTP
surface contract: a wrong-kind file POSTed to those endpoints must come
back as 400, never as 200 or 500.
Tests added:
- test_api_999.py: rejects_837_input, rejects_835_input
- test_api_277ca.py: rejects_835_input, rejects_837_input
- test_api_ta1.py: rejects_837_input, rejects_835_input
If a future PR relaxes any of those parser-level guards, the
corresponding regression lock fires immediately.
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).
Nora
Tyler