cultnet error contract: one message_invalid code replaces malformed_document_put/delete

Q6-A of docs/cultnet-error-contract-cut.md. message_invalid joins the closed
cultnet.error.v0 code enum (both schema copies), with details.field naming the
request field at fault ("message" when the whole frame cannot be read).
C# ForMessageInvalid, Rust CultNetErrorCode::MessageInvalid, TS
cultNetMessageInvalidError and Python message_invalid_error all write the same
captured bytes (error-vectors.json plus the C# and Rust cases). cultmesh-py and
cultmesh-ts refuse malformed puts and deletes with it; the C# subscription
servers use it for the missing subscriptionId.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/GameCult.Networking/CultNetDatabaseSubscriptionServer.cs

@@ -145,7 +145,7 @@ namespace GameCult.Networking
                 : request.SubscriptionId;
             if (string.IsNullOrWhiteSpace(subscriptionId))
             {
-                peer.SendCultNet(new CultNetErrorMessage { Error = "Database subscription requires a subscriptionId or messageId." });
+                peer.SendCultNet(CultNetErrorMessage.ForMessageInvalid("subscriptionId", "Database subscription requires a subscriptionId or messageId."));
                 return Task.CompletedTask;
             }