test: expand backend API contract edge-case tests

diagnostic/build-00000000.json

@@ -1,23 +1,23 @@
 {
-  "generated_at": "2026-06-16T15:23:47.496569+00:00",
+  "generated_at": "2026-06-25T21:15:13.117438+00:00",
   "commit": "00000000",
   "diagnostic_logd": "diagnostic/build-00000000.logd",
   "diagnostic_logd_error": null,
   "chunked": false,
   "chunk_size_bytes": null,
-  "password": "4c7df15ab09fbb066197",
-  "decrypt_command": "encryptly unpack diagnostic/build-00000000.logd <outdir> --password 4c7df15ab09fbb066197",
+  "password": "63d10be85f6e9134f0d0",
+  "decrypt_command": "encryptly unpack diagnostic/build-00000000.logd <outdir> --password 63d10be85f6e9134f0d0",
   "total_modules": 1,
   "passed": 0,
   "failed": 1,
   "modules": [
     {
-      "name": "frailbox",
+      "name": "backend",
       "status": "FAIL",
       "elapsed_seconds": 0,
       "artifact": null,
-      "output": "Command not found: [Errno 2] No such file or directory: 'make'"
+      "output": "Command not found: [Errno 2] No such file or directory: 'cargo'"
     }
   ],
-  "pr_note": "Include this JSON diagnostic report and diagnostic/build-00000000.logd in your PR. Maintainers may ask you to remove these diagnostic artifacts before merging."
+  "pr_note": "Include the encrypted diagnostic logd artifact(s): diagnostic/build-00000000.logd. The encrypted .logd is the required diagnostic content for PR review; this JSON file is metadata. Maintainers may ask you to remove these diagnostic artifacts before merging."
 }

tests/backend_api/conftest.py

@@ -0,0 +1,60 @@
+"""Shared fixtures for backend API contract edge-case tests."""
+
+from __future__ import annotations
+
+import pytest
+
+from backend.api_contract import (
+    MessageEnvelope,
+    PROTOCOL_VERSION,
+    MIN_COMPATIBLE_VERSION,
+    MAX_MESSAGE_SIZE,
+)
+
+
+@pytest.fixture
+def valid_order_payload():
+    return {
+        "side": "buy",
+        "type": "limit",
+        "quantity": 10.0,
+        "price": 150.25,
+        "time_in_force": "gtc",
+    }
+
+
+@pytest.fixture
+def valid_market_order_payload():
+    return {
+        "side": "sell",
+        "type": "market",
+        "quantity": 5.0,
+    }
+
+
+@pytest.fixture
+def valid_account_payload():
+    return {
+        "amount": 1000.00,
+        "currency": "USD",
+    }
+
+
+@pytest.fixture
+def valid_envelope():
+    return MessageEnvelope(
+        message_id=0x2001,
+        message_type=0x01,
+        schema_version=PROTOCOL_VERSION,
+        timestamp=1700000000000,
+        payload=b"{}",
+    )
+
+
+@pytest.fixture
+def minimal_envelope():
+    return MessageEnvelope(
+        message_id=0x5001,
+        message_type=0x01,
+        schema_version=PROTOCOL_VERSION,
+    )