Fixed #35393 -- Added excluded primary keys as hidden fields for the inline admin.

[KommuSoft]

Trac ticket number

ticket-35393

Branch description

Provide a concise overview of the issue or rationale behind the proposed changes.

The original main branch had an AdminInline where one can exclude an (editable) primary key, as a result the Django admin could no longer back-reference the forms to the original item.

The fix includes a hidden primary key HTML element to back reference correctly, and update the elements accordingly. See also https://stackoverflow.com/q/78353321/67579 and https://code.djangoproject.com/ticket/35393 .

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• [-] I have added or updated relevant docs, including release notes if applicable.
• [-] For UI changes, I have attached screenshots in both light and dark modes.

[timgraham]

Can you add a test?

[KommuSoft]

@timgraham : added a test.