Validate PrepareRequest.schema is 0 or 1 at construction

PrepareRequest was the only message with a user-settable schema field that
accepted arbitrary values. Invalid schema values would be packed into the
wire header and only rejected by the server, with no client-side error.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/messages/requests.py

@@ -144,6 +144,8 @@ class PrepareRequest(Message):
 
     def __post_init__(self) -> None:
         _check_uint64("db_id", self.db_id)
+        if self.schema not in (0, 1):
+            raise ValueError(f"schema must be 0 or 1, got {self.schema}")
 
     def _get_schema(self) -> int:
         return self.schema

tests/test_messages_requests.py

@@ -126,6 +126,16 @@ def test_default_schema_is_0_matching_go(self) -> None:
         header = Header.decode(encoded[:HEADER_SIZE])
         assert header.schema == 0
 
+    def test_rejects_invalid_schema(self) -> None:
+        """PrepareRequest should reject schema values other than 0 or 1."""
+        import pytest
+
+        with pytest.raises(ValueError, match="schema must be 0 or 1"):
+            PrepareRequest(db_id=1, sql="SELECT 1", schema=2)
+
+        with pytest.raises(ValueError, match="schema must be 0 or 1"):
+            PrepareRequest(db_id=1, sql="SELECT 1", schema=-1)
+
 
 class TestExecRequest:
     def test_schema_v0_for_small_params(self) -> None: