/// <summary>
/// For SQL, a nested filter represents an EXISTS clause with a join between
/// the parent entity being filtered and the related entity representing the
/// non-scalar filter input. This function:
/// 1. Defines the Exists Query structure
/// 2. Recursively parses any more(possibly nested) filters on the Exists sub query.
/// 3. Adds join predicates between the related entities to the Exists sub query.
/// 4. Adds the Exists subquery to the existing list of predicates.
/// </summary>
/// <param name="ctx">The middleware context</param>
/// <param name="filterField">The nested filter field.</param>
/// <param name="subfields">The subfields of the nested filter.</param>
/// <param name="predicates">The predicates parsed so far.</param>
/// <param name="queryStructure">The query structure of the entity being filtered.</param>
/// <exception cref="DataApiBuilderException">
/// throws if a relationship directive is not found on the nested filter input</exception>
private void HandleNestedFilterForSql(
    IMiddlewareContext ctx,
    InputField filterField,
    List<ObjectFieldNode> subfields,
    List<PredicateOperand> predicates,
    BaseQueryStructure queryStructure,
    ISqlMetadataProvider metadataProvider)
{
    string? targetGraphQLTypeNameForFilter = RelationshipDirectiveType.GetTarget(filterField);

    if (targetGraphQLTypeNameForFilter is null)
    {
        throw new DataApiBuilderException(
            message: "The GraphQL schema is missing the relationship directive on input field.",
            statusCode: HttpStatusCode.InternalServerError,
            subStatusCode: DataApiBuilderException.SubStatusCodes.UnexpectedError);
    }

    string nestedFilterEntityName = metadataProvider.GetEntityName(targetGraphQLTypeNameForFilter);

    // Validate that the field referenced in the nested input filter can be accessed.
    bool entityAccessPermitted = queryStructure.AuthorizationResolver.AreRoleAndOperationDefinedForEntity(
        entityIdentifier: nestedFilterEntityName,
        roleName: GetHttpContextFromMiddlewareContext(ctx).Request.Headers[CLIENT_ROLE_HEADER].ToString(),
        operation: EntityActionOperation.Read);

    if (!entityAccessPermitted)
    {
        throw new DataApiBuilderException(
            message: DataApiBuilderException.GRAPHQL_FILTER_ENTITY_AUTHZ_FAILURE,
            statusCode: HttpStatusCode.Forbidden,
            subStatusCode: DataApiBuilderException.SubStatusCodes.AuthorizationCheckFailed);
    }

    List<Predicate> predicatesForExistsQuery = new();

    // Create an SqlExistsQueryStructure as the predicate operand of Exists predicate
    // This query structure has no order by, no limit and selects 1
    // its predicates are obtained from recursively parsing the nested filter
    // and an additional predicate to reflect the join between main query and this exists subquery.
    SqlExistsQueryStructure existsQuery = new(
        GetHttpContextFromMiddlewareContext(ctx),
        metadataProvider,
        queryStructure.AuthorizationResolver,
        this,
        predicatesForExistsQuery,
        nestedFilterEntityName,
        queryStructure.Counter);

    // Recursively parse and obtain the predicates for the Exists clause subquery
    Predicate existsQueryFilterPredicate = Parse(ctx,
            filterField,
            subfields,
            existsQuery);
    predicatesForExistsQuery.Push(existsQueryFilterPredicate);

    // Add JoinPredicates to the subquery query structure so a predicate connecting
    // the outer table is added to the where clause of subquery.
    // For self-referencing relationships (e.g., parent/child hierarchy),...

</details>



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- Fixes Azure/data-api-builder#3028

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