[WIP] Memory leak

[Copilot]

Working on fixing the memory leak issue in DuckDBResult struct (#280).

Plan:

• Understand the problem: DuckDBResult struct implementing IDisposable causes memory leaks when used in using statements due to struct copying behavior
• Explore codebase to understand current usage patterns
• Build and test the project to ensure current state is working
• Remove IDisposable interface from DuckDBResult struct
• Add a static Destroy method or extension method to safely dispose DuckDBResult
• Update all existing code that calls Dispose() on DuckDBResult to use the new approach
• Update DuckDBDataReader.Close() method to properly destroy DuckDBResult instances
• Write focused tests to validate the fix prevents memory leaks
• Build and test to ensure no regressions
• Verify the solution addresses the original issue

Root Cause:
The issue is that DuckDBResult is a struct implementing IDisposable. When used in a using statement, C# creates a copy of the struct, and Dispose() is called on the copy, not the original. This means the native memory pointed to by IntPtr internal_data is not properly freed, causing memory leaks.

Solution:
Remove IDisposable from the struct and provide alternative mechanisms for resource cleanup that work correctly with struct semantics.

Fixes #280.

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