PINK: fix ctypes c_char_p null-byte truncation (INVALID_INTENT_PARSE)

_to_rust_bytes() centralises all Python→Rust JSON serialisation:
- _json_null_clean() strips U+0000 from all string values recursively
- ensure_ascii=True guarantees no 0x00 in output bytes
- All _json() call sites migrated; mode/verbosity now .encode("ascii")
- 9 null-safety unit tests added to TestRustBytesNullSafety

Root cause: ctypes.c_char_p silently truncates at first 0x00 byte,
causing serde_json "premature end of input at column 41" on EXIT intents
with BNB-USDT leverage values. Long-term fix: Rust FFI (ptr, len) pairs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prod/clean_arch/dita_v2/test_flaws.py

@@ -1479,3 +1479,80 @@ class TestNormalizeEngForTui:
         eng = {"open_positions": 0, "slot": {}}
         out = self._norm(eng)
         assert out["open_positions"] == [], "zero open_positions must become empty list"
+
+
+# ============================================================
+# _to_rust_bytes / _json_null_clean — null-byte safety
+# ============================================================
+
+class TestRustBytesNullSafety:
+    """_to_rust_bytes must never produce a 0x00 byte in its output.
+
+    Root cause: ctypes.c_char_p treats the first 0x00 as a C null terminator,
+    silently truncating the JSON before Rust's serde_json sees the full payload.
+    Reproduces the INVALID_INTENT_PARSE bug seen during BingX VST smoke test.
+    """
+
+    def _encode(self, payload):
+        from prod.clean_arch.dita_v2.rust_backend import _to_rust_bytes
+        return _to_rust_bytes(payload)
+
+    def _clean(self, obj):
+        from prod.clean_arch.dita_v2.rust_backend import _json_null_clean
+        return _json_null_clean(obj)
+
+    def test_no_null_bytes_in_normal_exit_intent(self):
+        payload = {
+            "action": "EXIT",
+            "asset": "BNB-USDT",
+            "leverage": 1.3465735902799727,
+            "target_size": 1.76,
+            "reference_price": 66337.09,
+            "limit_price": 0.0,
+            "trade_id": "t1",
+            "metadata": {},
+        }
+        encoded = self._encode(payload)
+        assert b"\x00" not in encoded, "EXIT intent must have no null bytes"
+
+    def test_no_null_bytes_when_string_contains_u0000(self):
+        """A string value containing \\u0000 must not produce a null byte in output."""
+        payload = {"event_id": "BX\x00data", "price": 100.0}
+        encoded = self._encode(payload)
+        assert b"\x00" not in encoded, "Null char in string must not produce null byte"
+
+    def test_no_null_bytes_in_seen_event_ids(self):
+        """seen_event_ids list is serialized with all other slot fields."""
+        payload = {"seen_event_ids": ["123", "456\x00789", "999"], "size": 1.76}
+        encoded = self._encode(payload)
+        assert b"\x00" not in encoded, "seen_event_ids with null chars must be clean"
+
+    def test_no_null_bytes_in_nested_metadata(self):
+        payload = {"metadata": {"venue_note": "order\x00ok", "id": 42}, "asset": "ENJ-USDT"}
+        encoded = self._encode(payload)
+        assert b"\x00" not in encoded, "Nested metadata null chars must be sanitized"
+
+    def test_output_is_valid_json(self):
+        import json
+        payload = {"action": "ENTER", "asset": "BNB-USDT", "leverage": 2.7, "seen_event_ids": ["e1"]}
+        encoded = self._encode(payload)
+        parsed = json.loads(encoded)
+        assert parsed["action"] == "ENTER"
+
+    def test_json_null_clean_replaces_null_in_string(self):
+        result = self._clean({"key": "val\x00ue"})
+        assert "\x00" not in result["key"]
+        assert "val" in result["key"]
+
+    def test_json_null_clean_recursion(self):
+        obj = {"nested": {"list": ["a\x00b", 1, {"deep": "x\x00y"}]}}
+        cleaned = self._clean(obj)
+        assert "\x00" not in cleaned["nested"]["list"][0]
+        assert "\x00" not in cleaned["nested"]["list"][2]["deep"]
+
+    def test_normal_ascii_payload_roundtrips_intact(self):
+        import json
+        payload = {"action": "EXIT", "asset": "BTC-USDT", "leverage": 1.5, "size": 0.001}
+        encoded = self._encode(payload)
+        assert json.loads(encoded)["asset"] == "BTC-USDT"
+        assert json.loads(encoded)["leverage"] == 1.5