Fix error handling in SimpleExecuteRequest to return actual SQL error messages

[caohai]

Description

Fixes two bugs in HandleSimpleExecuteRequest:

1. SQL error messages not returned: Queries with SQL errors (e.g., invalid table names) returned generic "Query has no results to return" instead of actual error details. Fixed by collecting error messages via BatchMessageSent events and checking batch.HasError in the completion callback.

2. Queries without result sets treated as errors: Valid DML/DDL statements (UPDATE, DELETE, CREATE TABLE) incorrectly returned errors. Fixed by returning success with empty result arrays when resultSets.Count == 0.

What these two fixes look like for query execution in GHCP:

Code Changes Checklist

• New or updated unit tests added (2 integration tests)
• All existing tests pass (dotnet test)
• Code follows contributing guidelines
• Logging/telemetry updated if relevant
• No protocol or behavioral regressions

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