feat(sp35): /api/parse-835 envelope + empty-claims guards

Mirror of the parse-837 SP35 guards. Same defense-in-depth shape:
tokenize first, reject anything whose ST01 doesn't start with '835'
(400 'Mismatched file kind'), and after parse refuse to persist a
batch with zero CLP segments (400 'No claims parsed').

Reuses the _transaction_set_id_from_segments helper added by the
parse-837 commit.

New tests in tests/test_api_835.py:
- test_parse_835_endpoint_rejects_837_input (was failing, now green)
- test_parse_835_endpoint_rejects_empty_envelope (was failing, now green)
- test_parse_835_endpoint_happy_path_still_works (regression guard)

backend/tests/test_api_835.py

@@ -13,6 +13,7 @@ from cyclone.store import store as global_store
 
 FIXTURE = Path(__file__).parent / "fixtures" / "co_medicaid_835.txt"
 UNBALANCED = Path(__file__).parent / "fixtures" / "unbalanced_835.txt"
+FIXTURE_837P = Path(__file__).parent / "fixtures" / "co_medicaid_837p.txt"
 
 
 @pytest.fixture
@@ -337,3 +338,117 @@ def test_prodfile_round_trip_persists_separately(client: TestClient):
     # No duplicate PCNs survived the dedup; sanity check on persistence.
     pcns = [r["claimId"] for r in all_remits if r["claimId"]]
     assert len(pcns) == len(set(pcns)), "duplicate PCN across batches would be a persistence bug"
+
+
+# --------------------------------------------------------------------------- #
+# SP35: parse-835 input guards (mirror of the parse-837 guards)
+# --------------------------------------------------------------------------- #
+#
+# Before SP35, /api/parse-835 had the same silent-corruption shape as
+# /api/parse-837: any file with a parseable ISA envelope was accepted, and
+# the 835 parser returned claims=[] when the file had no CLP segments.
+# This produced empty 835 batches and bogus rows on the History tab.
+# These tests are the server-layer regression locks for the 835 endpoint.
+
+
+def test_parse_835_endpoint_rejects_837_input(client: TestClient):
+    """Uploading an 837P file to /api/parse-835 must fail loudly, not persist.
+
+    Repro for the symmetric bug: user drops an 837P file on the Upload page
+    while the dropdown still says "835" (Upload.tsx default). Before SP35 the
+    endpoint accepted it, ran it through the 835 parser (which found zero
+    CLP segments), and persisted an empty claims=[] batch. SP35 closes the
+    door at the server so the UI bug becomes cosmetic instead of
+    data-corrupting.
+    """
+    text_837p = FIXTURE_837P.read_text()
+    # Sanity check: the fixture really is an 837P file. If this ever flips,
+    # the test would still pass for the wrong reason.
+    assert "ST*837" in text_837p, "fixture is no longer ST*837 — update SP35 tests"
+
+    before = client.get("/api/batches", headers={"Accept": "application/json"}).json()
+    total_before = before.get("total", len(before.get("items", [])))
+
+    resp = client.post(
+        "/api/parse-835",
+        files={"file": ("co_medicaid_837p.txt", text_837p, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+
+    assert resp.status_code == 400, resp.text
+    body = resp.json()
+    assert body["error"] == "Mismatched file kind"
+    assert body["expected"] == "835"
+    assert body["detected_st"].startswith("837")
+
+    after = client.get("/api/batches", headers={"Accept": "application/json"}).json()
+    total_after = after.get("total", len(after.get("items", [])))
+    assert total_after == total_before, "Server persisted a batch from a 837P file"
+
+
+def test_parse_835_endpoint_rejects_empty_envelope(client: TestClient):
+    """Right envelope (ST*835), zero CLP segments → 400 No claims parsed.
+
+    Synthetic input: a complete ISA/GS/ST envelope with a BPR + TRN, a
+    closing SE/GE/IEA, and no CLP loops. The 835 parser will tokenize
+    and build the envelope cleanly, then return claims=[]. SP35 must
+    surface this as a 400 with error="No claims parsed" and must not
+    persist a batch.
+    """
+    # Bare 835 envelope — no LX/CLP loops. A real X12 835 with no claims
+    # is unusual but possible (header-only test file or a cancelled run).
+    # The right behavior is to reject, not to silently persist.
+    synthetic = (
+        "ISA*00*          *00*          *ZZ*SENDER         *ZZ*RECEIVER       "
+        "*260617*1937*^*00501*000000001*0*P*:~"
+        "GS*HP*SENDER*RECEIVER*20260706*1937*1*X*005010X221A1~"
+        "ST*835*0001~"
+        "BPR*I*100.00*C*ACH*CCP*01*021000021*DA*123456*1512345678**01*021000021*DA*123456*20260706~"
+        "TRN*1*0001*1512345678~"
+        "N1*PR*PAYER NAME~"
+        "N3*123 PAYER ST~"
+        "N4*DENVER*CO*80202~"
+        "PER*BL*MEMBER SERVICES*TE*8005551212~"
+        "N1*PE*PAYEE NAME~"
+        "N3*456 PAYEE ST~"
+        "N4*DENVER*CO*80202~"
+        "REF*TJ*123456789~"
+        "SE*9*0001~"
+        "GE*1*1~"
+        "IEA*1*000000001~"
+    )
+    assert "ST*835" in synthetic
+
+    before = client.get("/api/batches", headers={"Accept": "application/json"}).json()
+    total_before = before.get("total", len(before.get("items", [])))
+
+    resp = client.post(
+        "/api/parse-835",
+        files={"file": ("empty_835.txt", synthetic, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+
+    assert resp.status_code == 400, resp.text
+    body = resp.json()
+    assert body["error"] == "No claims parsed"
+
+    after = client.get("/api/batches", headers={"Accept": "application/json"}).json()
+    total_after = after.get("total", len(after.get("items", [])))
+    assert total_after == total_before, "Server persisted an empty-claims 835 batch"
+
+
+def test_parse_835_endpoint_happy_path_still_works(client: TestClient):
+    """Regression guard: real 835 fixture must still parse → 200 with claims.
+
+    Sits next to the new SP35 rejection tests so any future tightening of
+    the guards that accidentally blocks the happy path fails here loudly.
+    """
+    text = FIXTURE.read_text()
+    resp = client.post(
+        "/api/parse-835",
+        files={"file": ("co_medicaid_835.txt", text, "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert resp.status_code == 200, resp.text
+    body = resp.json()
+    assert body.get("summary", {}).get("total_claims", 0) >= 1