feat: add QueryableProjectionNullHandling option to skip projection null handling

[fdipuma]

Description

Adds a QueryableProjectionNullHandling mapper option that lets IQueryable<T> projection mappings skip null handling, for relational providers (e.g. EF Core) where the projection lambda is translated to SQL and never executed in memory.

This implements part 3 of #2317 and should close:

• Fixes IQueryable projections have unnecessary null checks #2350: nullable navigation -> non-nullable DTO no longer emits unnecessary SQL CASE blocks nor reports RMG002 ("no accessible parameterless constructor").
• Fixes Unexpected IQueryable mapping when project's Nullable flag is set to "disable" #1293: projection output is now identical under #nullable enable/disable.

Option

public enum QueryableProjectionNullHandling
{
    NullSafe = 0, // default, current null-safe behaviour
    Ignore,       // skip null handling in queryable projections
}

Set at mapper or assembly (MapperDefaults) level:

[Mapper(QueryableProjectionNullHandling = QueryableProjectionNullHandling.Ignore)]
public static partial class CarMapper
{
    public static partial IQueryable<CarDto> ProjectToDto(this IQueryable<Car> q);
}

Under Ignore, projection member access is emitted directly (x.Owner.Address.City, x.IntValue.Value for nullable value types) with no ?., ternary != null ? … : default, coalesce, or null fallback; the RMG089 nullability diagnostic is suppressed. Nullable value-type members are unwrapped so the generated expression compiles. The default NullSafe keeps today's behaviour unchanged (non-breaking).

note

The related idea from #2317 of honoring a conversion operator's own nullable annotations in non-projection mappings is intentionally not included here: it turned out to require changes to the shared member-mapping null-guard path and is best handled as a separate, focused PR. This PR is limited to the queryable-projection option.

Checklist

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