[BUG] `salt-minion 3007.0 (Chlorine)` cannot use `file.append` with a single source (expecting a list object not a string)

Description
A clear and concise description of what the bug is.

Trying to salt-call state.sls someAppendState causes AttributeError: 'list' object has no attribute 'decode'

Setup
(Please provide relevant configs and/or SLS files (be sure to remove sensitive info. There is no general set-up of Salt.)

Please be as specific as possible and give set-up details.

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
  KVM and desktop. All are impacted.
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD
• classic packaging
• onedir packaging
• used bootstrap to install

Steps to Reproduce the behavior
(Include debug logs if possible and relevant)

Expected behavior
A clear and concise description of what you expected to happen.

An example state in a file on its own which is falling over:

appender:
  file.append:
    - name: /a
    - source:
      - salt://b

Screenshots
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Versions Report

Salt Version:
          Salt: 3007.0
 
Python Version:
        Python: 3.11.8 (main, Feb 12 2024, 14:50:05) [GCC 13.2.1 20230801]
 
Dependency Versions:
          cffi: 1.16.0
      cherrypy: unknown
      dateutil: 2.9.0
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 3.1.3
       libgit2: Not Installed
  looseversion: 1.3.0
      M2Crypto: 0.40.1
          Mako: 1.3.2
       msgpack: 1.0.5
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 23.2
     pycparser: 2.21
      pycrypto: Not Installed
  pycryptodome: 3.12.0
        pygit2: Not Installed
  python-gnupg: 0.5.2
        PyYAML: 6.0.1
         PyZMQ: 25.1.2
        relenv: Not Installed
         smmap: Not Installed
       timelib: 0.3.0
       Tornado: 6.4
           ZMQ: 4.3.5
 
Salt Package Information:
  Package Type: Not Installed
 
System Versions:
          dist: arch rolling n/a
        locale: utf-8
       machine: x86_64
       release: 6.7.8-arch1-1
        system: Linux
       version: Arch Linux rolling n/a

Additional context
Add any other context about the problem here.

Archlinux 6.7.8-arch1-1

The payload names the salt-call state.sls someAppendState entry point, the file.append state, and an example SLS with a single source; no source file or test is identified. Start by reproducing the AttributeError with that state, then trace how file.append handles the source value. Done means a single-source state runs successfully without the reported decode error.