Added BsonRefId<T> to allow explicit DbRefs in LINQ expressions

[rklfss]

With this PR, a LINQ expression helper type BsonRefId<T> is introduced that allows explicit setting of DbRef objects in LINQ expressions, for example in UpdateMany.

db.GetCollection<A>()
    .UpdateMany(
        x => new A
        {
            Id = x.Id,
            Bref = new BsonRefId<B>(100), // instead of `new B { }` which would be included inline
        },
        x => x.Id == 11);

Resolves #2739

[JKamsker]

@codex review

[chatgpt-codex-connector]

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[JKamsker]

@codex review

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Reviewed commit: 2df20d84d2

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[chatgpt-codex-connector]

Restore public DbRef configuration for custom mappers

The commit makes MemberMapper.IsDbRef read-only and ties it to the new internal DbRefCollectionName, which means external code can no longer mark a member as a DbRef in ResolveMember or other custom mapping hooks. This is a breaking change for consumers who previously set member.IsDbRef = true (and optionally supplied custom serialize/deserialize) without using [BsonRef] or EntityBuilder.DbRef. Because DbRefCollectionName is internal, there is now no public way to achieve the same configuration, so existing mapping code will fail to compile or silently lose DbRef behavior. Consider keeping a public setter or exposing a public way to set the collection name.

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[rklfss]

@JKamsker How to handle this? I understand the point, but don't see how to just set the IsDbRef flag (in ResolveMember?) would lead to something useful. Doing this manual currently requires also to set a pretty complex Serialize/Deserialize method. I would even go further and try to make the Entity/Member mappers themselves internal, letting the generic ResolveMember method use some builder approach like it exists for the EntityBuilder.

[JKamsker]

Thanks for the pr!