fix(core): use property initializers in Ivf DTOs (compatibility safe)

[takurot]

Summary

Resolved CS8618 warnings in 'IvfFlatVectorIndex.cs' by using property initializers.
Initially considered 'required', but switched to initializers to ensure backward compatibility with older snapshots (preventing JsonException on missing fields).

Changes

• Replaced 'required' modifier with default value initialization ('= new()', '= null!').
• Added regression test 'Load_MissingFields_ShouldHandleGracefully' to verify JSON compatibility.

Verification

• 'dotnet build' passes without warnings.
• 'dotnet test' passes (including new compatibility test case).