{{ dirname }} placeholder not set for `public_folder` when initially creating a entry in a nested collection

### Describe the bug

When creating an entry in a nested collection, the `{{ dirname }}` placeholder appears to not be set for `public_folder`.

Using the following collection:
```yaml
- label: Dokumentmappar
label_singular: Dokumentmapp
name: documents
folder: src/dokument/
create: true
nested:
summary: "{{ fields.title }}"
depth: 10
fields:
- {label: Titel, name: title}
- {label: Anteckningar, name: body, widget: markdown, required: false}
- {name: permalink, widget: hidden, default: false}
- label: Dokument
name: files
widget: list
summary: "{{ fields.file }}"
field:
name: file
widget: file
media_folder: "/src/dokument/{{ dirname }}"
public_folder: "/dokument/{{ dirname }}"
```
this causes the file paths for `public_folder` to get improperly set, despite the files getting placed correctly by the `media_folder` attribute.

### To Reproduce

1. Use the above collection configuration.
2. Create a nested entry with files attached.
3. Notice that the filepaths set in the .md file is `/dokument/filename.ext` instead of `/dokument/nested/filename.ext`
4. Notice that the files are in the correct position, and editing the nested entry yields the correct paths.

### Expected behavior

Files created should have their filepaths correctly set.

### Screenshots

n/a

### Applicable Versions:

- Decap CMS version: 3.10.1
- Git provider: GitLab
- OS: Ubuntu
- Browser version: Mozilla Firefox 148.0
- Node.JS version: v22.22.1

### CMS configuration

https://git.ludd.ltu.se/lulespexet/www/-/blob/master/src/admin/config.yml

### Additional context

Example commit: https://git.ludd.ltu.se/lulespexet/www/-/commit/ba5b2f6ae8cd03081fe3ab21ecd813f911d7dda5

Start with the linked CMS configuration and reproduce creation of a nested entry using the nested collection and file widget settings shown in the issue. Compare the generated Markdown path with the path after editing the entry; done means the initial creation uses the nested directory in the public file path, matching the media location.

Written by the indexing model from the issue text.