Fix reducer errors to return message not stack trace (#3965)

# Description of Changes
This PR fixes a C# reducer error-reporting issue where exceptions thrown
in reducers were returned to clients with full stack traces, as reported
in [#3959](#3959).

- Updated the C# reducer host-call error serialization to return only
`Exception.Message` (instead of `Exception.ToString()`), preventing
server-side stack traces from being leaked to clients.
- Added a C# regression-test client assertion to validate reducer
failures do not contain stack traces:
  - `exception.Message` matches the thrown message exactly
- the message does not contain newlines or `" at "` stack-trace markers

# API and ABI breaking changes
None.
- No schema or wire-format changes.
- Behavior change is limited to the *contents* of reducer error strings
returned to clients (stack traces removed).

# Expected complexity level and risk
2 - Low
- Change is isolated to C# reducer error formatting plus a regression
assertion.
- Removes unintended information exposure without affecting reducer
success paths.

# Testing
- [X] Ran C# regression-tests client; verified reducer error message is
preserved and stack trace is not present.

Author: rekhoff

crates/bindings-csharp/Runtime/Internal/Module.cs

@@ -312,7 +312,7 @@ BytesSink error
         }
         catch (Exception e)
         {
-            var error_str = e.ToString();
+            var error_str = e.Message ?? e.GetType().FullName;
             var error_bytes = System.Text.Encoding.UTF8.GetBytes(error_str);
             error.Write(error_bytes);
             return Errno.HOST_CALL_FAILURE;

sdks/csharp/examples~/regression-tests/client/Program.cs

@@ -12,6 +12,7 @@
 
 const string HOST = "http://localhost:3000";
 const string DBNAME = "btree-repro";
+const string THROW_ERROR_MESSAGE = "this is an error";
 
 DbConnection ConnectToDB()
 {
@@ -94,6 +95,7 @@ void OnConnected(DbConnection conn, Identity identity, string authToken)
     {
         Log.Info($"Got OnUnhandledReducerError: {exception}");
         waiting--;
+        ValidateReducerErrorDoesNotContainStackTrace(exception);
         ValidateBTreeIndexes(ctx);
         ValidateNullableVecView(ctx);
     };
@@ -174,6 +176,17 @@ void ValidateNullableVecView(
     }
 }
 
+void ValidateReducerErrorDoesNotContainStackTrace(Exception exception)
+{
+    Debug.Assert(
+        exception.Message == THROW_ERROR_MESSAGE,
+        $"Expected reducer error message '{THROW_ERROR_MESSAGE}', got '{exception.Message}'"
+    );
+    Debug.Assert(!exception.Message.Contains("\n"), "Reducer error message should not contain newline");
+    Debug.Assert(!exception.Message.Contains("\r"), "Reducer error message should not contain newline");
+    Debug.Assert(!exception.Message.Contains(" at "), "Reducer error message should not contain stack trace");
+}
+
 void OnSubscriptionApplied(SubscriptionEventContext context)
 {
     applied = true;
@@ -194,7 +207,7 @@ void OnSubscriptionApplied(SubscriptionEventContext context)
 
     Log.Debug("Calling ThrowError");
     waiting++;
-    context.Reducers.ThrowError("this is an error");
+    context.Reducers.ThrowError(THROW_ERROR_MESSAGE);
 
     Log.Debug("Calling InsertResult");
     waiting++;