Enable nullable reference types

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Problem

Both DBTools.csproj and DBToolsUnitTest.csproj set <Nullable>disable</Nullable>, missing compile-time null safety that is standard for modern .NET libraries.

Proposed solution

1. Enable <Nullable>enable</Nullable> in DBTools.csproj
2. Fix nullability warnings in public API surface first (Core/, Abstractions/, Configuration/)
3. Enable in test project after library compiles cleanly
4. Add #nullable enable incrementally per file if a big-bang migration is too large

Priority areas:

• DbConfiguration (config values from JSON)
• LINQ expression translation return types
• GenericObject property arrays

Acceptance criteria

• Nullable enabled in library project
• Build succeeds with no/nullable warnings suppressed only where justified
• Public API annotations (?, !) are accurate

Notes

Can be done incrementally with warningsAsErrors optional in a follow-up.