Fix reducer errors to return message not stack trace

[rekhoff]

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.

• 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

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

[jdetter]

This one seems simple enough, thanks for the regression test 👍 I haven't tested this myself but this seems quite trivial