26.2: referencing is a direct dependency, but only gets pulled in via jsonschema

# Bug report

Hi 👋
I'm maintaining this project as a package for Arch Linux.

When building 26.2 I noticed, that referencing is used, but is not declared as direct dependency:

https://github.com/canonical/cloud-init/blob/a6d999ae517d62d2d557d7b2b4d8dd403abdde35/cloudinit/config/schema.py#L666-L667

It seems referencing is a transitive dependency pulled in by jsonschema.

## Steps to reproduce the problem

Look at the declared dependencies and the above import.

## Environment details
- Cloud-init version: 26.2
- Operating System Distribution: Arch Linux
- Cloud provider, platform or installer type: built from source

## cloud-init logs

n/a

Start with cloudinit/config/schema.py at the referenced lines and inspect the project's declared dependency metadata for the missing direct dependency. Confirm that referencing is declared independently of jsonschema and verify the package can be built or installed with the corrected dependency set.

Written by the indexing model from the issue text.