fix: Allow legacy PlaceholderField to be deconstructed

[stefanw]

Description

A recent commit has removed the deconstruct method on the dummy PlaceholderField.

This breaks migrations with the error:

TypeError: "PlaceholderField" missing 1 required positional argument: slotname.

This PR adds the deconstruct method back and allows migrations reconstruct previous model state in migrations.

Checklist

• [x] I have opened this pull request against main
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined the channel #pr-reviews on our Discord Server to find a “pr review buddy” who is going to review my pull request.

Summary by Sourcery

Bug Fixes:

• Re-add deconstruct method to include the slotname argument and prevent migration errors when using PlaceholderField

[sourcery-ai[bot]]

Reviewer's Guide

Reintroduces the deconstruct method on PlaceholderField and retains the slotname attribute to ensure legacy migrations can reconstruct the field without errors.

Updated Class Diagram for PlaceholderField

classDiagram
    class ForeignKey {
        <<Django Model Field>>
        +deconstruct(): tuple
    }
    PlaceholderField --|> ForeignKey
    class PlaceholderField {
        +slotname: string
        +__init__(slotname, *args, **kwargs)
        +deconstruct(): tuple
    }

File-Level Changes

Change	Details	Files
Persist slotname on placeholder fields	Assign self.slotname = slotname in the field constructor Maintain existing to and on_delete kwargs	cms/models/fields.py
Restore the deconstruct method for migrations	Define deconstruct to call super().deconstruct() Inject slotname back into the returned kwargs Return the modified (name, path, args, kwargs) tuple	cms/models/fields.py

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[fsbraun]

@stefanw Oh, yes, indeed!

Shall we also add a system_check_removed_details message, e.g.:

    system_check_removed_details = {
        "msg": (
            "PlaceholderField is for django CMS versions below 4 only. It may only be used inside migrations."
        ),
        "hint": "Use PlaceholderRelationField instead.",
        "id": "cms.E001",
    }