feat(sp35): regression locks on parse-999/277ca/ta1 envelope guards

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.

backend/tests/test_api_277ca.py

@@ -167,3 +167,48 @@ class TestInboxPayerRejectedLane:
         # The rejected lane (999 envelope) must be empty — we haven't
         # uploaded a 999, so this claim isn't there.
         assert "c1" not in [c["id"] for c in lanes["rejected"]]
+
+
+# --------------------------------------------------------------------------- #
+# SP35: parse-277ca envelope regression lock
+# --------------------------------------------------------------------------- #
+#
+# The 277CA parser already raises CycloneParseError("Expected ST*277 or
+# ST*277CA, got ST*<other>") when fed a file with the wrong ST envelope
+# (parse_277ca.py line 298). This regression lock confirms the HTTP
+# surface converts that error into a 400 (never 200, never 500).
+
+
+def test_parse_277ca_endpoint_rejects_835_input(client: TestClient):
+    """Posting an 835 file to /api/parse-277ca must surface 400, not 200."""
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_835.txt"
+    text = wrong_kind.read_text()
+    assert "ST*835" in text  # sanity check on the fixture
+
+    resp = client.post(
+        "/api/parse-277ca",
+        files={"file": ("co_medicaid_835.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    body = resp.json()
+    assert "error" in body
+    # The parser-level message must survive (mentions "ST" and the
+    # expected vs actual set id).
+    detail = body.get("detail", "")
+    assert "ST" in detail and ("277" in detail or body["error"] == "Parse error"), body
+
+
+def test_parse_277ca_endpoint_rejects_837_input(client: TestClient):
+    """Posting an 837P file to /api/parse-277ca must surface 400, not 200."""
+    wrong_kind = Path(__file__).parent / "fixtures" / "co_medicaid_837p.txt"
+    text = wrong_kind.read_text()
+    assert "ST*837" in text  # sanity check
+
+    resp = client.post(
+        "/api/parse-277ca",
+        files={"file": ("co_medicaid_837p.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 400, resp.text
+    assert "error" in resp.json()