[ActiveStorage] Ensure that the `_blob` association is properly loaded when attaching `::One`

Consider a model with `One` and `Many` attachments configured:

    class User < ActiveRecord::Base
      has_one_attached :avatar
      has_many_attached :highlights
    end

=== One Attachment

After attaching `One` attachment (`:avatar`), we can see that the associated
`_blob` record (`:avatar_blob`) still returns as `nil`.

    user.avatar.attach(blob)
    user.avatar_attachment.present?  => true
    user.avatar_blob.present?        => false    # Incorrect!

This is a false negative. It happens because after the attachment and blob
are built:

  1. The record already has its `_blob` association loaded, as `nil`
  2. the `::Attachment` is associated with the record but the `::Blob` only gets
    associated with the `::Attachment`, not the record itself

In reality, the blob does in fact exist. We can verify this as follows:

    user.avatar.attach(blob)
    user.avatar_attachment.blob.present?    => true  # Blob does exist!

The fix in this change is to simply assign the `::Blob` when assigning
the `::Attachment`. After this fix is applied, we correctly observe:

    user.avatar.attach(blob)
    user.avatar_attachment.present?  => true
    user.avatar_blob.present?        => true    # Woohoo!

=== Many Attachments

We don't see this issue with `Many` attachments because the `_blob` association
is already loaded as part of attaching more/newer blobs.

    user.highlights.attach(blob)
    user.highlights_attachments.any?    => true
    user.highlights_blobs.any?          => true

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