Add `allMatch` condition for to-many relations (linkMany / backlinkMany)

Is there an existing issue?

• I have searched existing issues

Use case

My data model has a Congress linked to many Sessions, and each Session
linked to many Members:

@Entity()
class Congress {
  ...
  final ToMany<Session> sessions; 
}
@Entity()
class Session {

  ... 
  final ToMany<Member> members;
  @Backlink()
  final ToMany<Congress> congress;
} 

@Entity(realClass: Member)
abstract class Member with _$Member {
  factory Member({
    @Backlink() required ToMany<Session> sessions, 
    ...
  }) = _Member;
}

I want to retrieve all Members that are only ever associated with a single,
specific Congress (as opposed to members who also participate in other
congresses). Today I can build a query that walks the relation chain from
Member up to Congress using backlinkMany, but it only expresses "at
least one" (any-match):

  final builder = memberBox.query();
  builder
      .backlinkMany(Session_.members)
      .backlinkMany(Congress_.sessions, Congress_.id.equals(congressId));
  final query = builder.build();

This matches members who are linked to congressId among others, but there
is no way to express "every Congress reachable through this member's
sessions is congressId", i.e. an all-match version of this condition. In this use case, the congressId is a unique String business ID not the ObjectBox int id.

Proposed solution

Add an all-match version of linkMany/backlinkMany, for example
linkManyAll/backlinkManyAll, that only matches an object if all of its
related objects (for a to-many relation) satisfy the given
condition:

  final builder = memberBox.query();
  builder
      .backlinkManyAll(Session_.members)
      .backlinkManyAll(Congress_.sessions, Congress_.id.equals(congressId));
  final query = builder.build();

This would match members whose entire set of sessions only ever points back
to congressId, allowing for a single objectBox query.

Describe alternatives you've considered

• Loading all matching members and re-checking their related sessions in
  Dart code. This would work but adds consequent post-processing of query results.
• Denormalizing by storing the list of unique congressId in Member and perform an equality matching on a list ["<congressId">] . This adds some complexity to the initial data write but does simplify querying.

Start with the query-builder entry points used by memberBox.query(), especially linkMany and backlinkMany, and trace how their to-many conditions are evaluated. Define the all-match behavior for nested linkMany/backlinkMany relations, including the requested Congress and Session example, then verify that a query only matches when every reachable related object satisfies the condition.