fix(mongo): move $fulltext from $and to top level

[funduck]

Closes #4065

[B4nan]

thanks!

[B4nan]

lets mention the $fulltext operator directly here instead of "full text searches"

[B4nan]

can't say i like the solution, but i don't know any better right now. note that you introduced 4 levels of branching, you will need ~4 tests to cover what this does, not just one.

also lets move this into a private method (we should do the same with the rest of the fulltext code, this method is crazy long and complex)

[funduck]

Oh, I see you fixed those things by yourself. I was going to, but then started thinking about that limitation "Full text search is only supported on the top level of the query object", why is it so?
And it looks like mongodb (I checked 4.2.6 and latest) allows $text to appear in nested conditions. You can check it with these tests

  test('positive $text in nested conditions', async () => {
    const booksRepository = orm.em.getRepository(Book);
    const books = await booksRepository.find({ $or: [
      {$text: {$search: 'Bible'} },
      {title: {$regex: 'Something'} },
    ]} as unknown as FilterQuery<Book>);
    expect(books.length).toBe(1);
  })

  test('negative $text in nested conditions', async () => {
    const booksRepository = orm.em.getRepository(Book);
    const books = await booksRepository.find({ $or: [
      {$text: {$search: 'Something'} },
      {title: {$regex: 'Bible'} },
    ]} as unknown as FilterQuery<Book>);
    expect(books.length).toBe(1);
  })

So I thought is it really necessary to forbid $fulltext in nested objects?
Of course there are some limitations to use of $text, for example in $or mongo won't allow non-indexed fields together with $text and this kind of query will fail

{$or: [{$text:{$search: 'some full text request'}}, {'someNonIndexedField': 42}]}

If you think it's a good idea, I could work on that

[B4nan]

This came in as a community PR, feel free to improve it, the limitation indeed sounds quite artificial.