entry manager per field instance

[i-just]

Description

Issue:
Saving nested element changes against the owner’s draft doesn’t work as expected with multi-instance fields.

Screen.Recording.2025-04-04.at.11.39.37.mov

Steps to reproduce:

• CMS 5.6+ and CKEditor 4.6.0
• create a text entry type; it can contain simply the default title
• create a CKEditor config with the “New entry” button and select text entry type under “Entry Types”
• create a CKEditor field that uses this config
• create a section (all default settings) with a new entry type that has two instances of the CKEditor field from the previous step included twice
• create an entry in this section, in each CKE field add one nested entry & fully save everything
• edit the nested entry in the second cke field on the page, save and then save the root entry - changes are discarded when the root element is saved
• swap the fields around in the entry type
• edit the nested entry in the second cke field on the page, save and then save the root entry - changes are discarded when the root element is saved
• if you edit whichever field is first on the page, it’ll work as expected, regardless of whether it has an overwritten handle or not

Solution:
Create nested element manager on per-instance basis.

Related issues

n/a, reported by @tommysvr