Fall back to the unversioned banner slug

[MickLesk]

✍️ Description

get_header() builds the banner slug from APP at call time. Three VM scripts set APP a second time with the release before drawing, so they ask for a banner that was never generated — the generator keys on the first APP= line:

Script	second APP=	slug requested	upstream
vm/almalinux-vm.sh	AlmaLinux ${var_version} VM	almalinux10vm	404
vm/debian-vm.sh	Debian ${var_version}	debian13	404
vm/ubuntu-vm.sh	Ubuntu ${var_version} VM	ubuntu26.04vm	404

🔗 Related Issue

Fixes community-scripts/ProxmoxVE#17383

✅ Prerequisites (X in brackets)

• Self-review completed
• Tested thoroughly – see below
• No security risks
• API.txt regenerated – _cs_header_slugs is not exported through a loader snapshot; get_header keeps its signature

🧪 Testing

_cs_header_slugs extracted from the file and run against 11 cases — all pass:

"AlmaLinux 10 VM"       -> almalinux10vm almalinux10 almalinux
"Debian 13"             -> debian13 debian
"Ubuntu 26.04 VM"       -> ubuntu26.04vm ubuntu26.04 ubuntu
"Home Assistant OS"     -> homeassistantos
"TrueNAS Scale VM"      -> truenasscalevm truenasscale
"Mikrotik RouterOS CHR" -> mikrotikrouteroschr
"Debian v13"            -> debianv13 debian
"Foo LXC"               -> foolxc foo
""                      -> (nothing)

get_header end to end against local fixtures:

"Debian 13"         -> DEBIAN-BANNER  rc=0   (fell back to debian)
"Alpine"            -> ALPINE-BANNER  rc=0   (direct hit)
"Home Assistant OS" -> <nothing>      rc=1   (no fallback, as intended)
cache written under the resolved slug: headers/vm/debian
second call serves from cache with the fixture removed: DEBIAN-BANNER rc=0

Not run on a Proxmox host. bash -n passes.

🛠️ Type of Change (X in brackets)

• 🐞 Bug fix

[github-actions]

Try this branch

The engine and the scripts resolve independently, so a production script can
be run against the engine from this PR by setting one variable:

COMMUNITY_SCRIPTS_CORE_URL=https://raw.githubusercontent.com/community-scripts/core/fix/vm-banner-versioned-app \
bash -c "$(curl -fsSL https://raw.githubusercontent.com/community-scripts/ProxmoxVED/main/ct/debian.sh)"

Swap ct/debian.sh for whatever exercises the change.

This PR touches pve/, so it needs a Proxmox VE host.

Run a script from a fork as well

curl -fsSL https://raw.githubusercontent.com/community-scripts/core/fix/vm-banner-versioned-app/tools/run.sh |
  bash -s -- https://raw.githubusercontent.com/YOU/ProxmoxVED/your-branch ct/debian.sh \
             https://raw.githubusercontent.com/community-scripts/core/fix/vm-banner-versioned-app

Note that run.sh is reached through a pipe, so the script it starts inherits
an exhausted stdin. Whiptail is fine — it opens /dev/tty — but a plain read
would see EOF. The single-variable form above does not have that problem.

Useful flags while testing

dev_mode=net logs every engine fetch with status and duration, which is the
quickest way to confirm the branch is really being used. dev_mode=keep stops a
failed build from deleting the container along with the evidence.