test(e2e): dual-detect ubuntu image version (24.04/26.04) during gold-image transition

After mulga gold image bumped to v6-fastboot (stages ubuntu-26.04 / resolute
instead of ubuntu-24.04 / noble), all consumers that hardcoded 24.04 break on
the new image. Make them detect at runtime so v3 and v6 envs both pass.

- tests/e2e/run-e2e.sh: DescribeImages filter accepts both
  ami-ubuntu-{26,24}.04-<arch>, then derives IMAGE_NAME from the match.
- tests/e2e/run-pseudo-multinode-e2e.sh: prefer ~/images/ubuntu-26.04.img,
  fall back to ubuntu-24.04.img; pass detected version to spx import.
- tests/e2e/single/image_test.go: walk candidate catalog names (26.04 first,
  then 24.04), stop on the first one DescribeImages can resolve.
- .github/workflows/e2e-baremetal.yml: download resolute (26.04) into
  BM_ISO_CACHE; update BM_UBUNTU_IMAGE env var path.
- docs/terraform-workbooks/{bastion-private-subnet,s3-webapp,nginx-webserver}:
  AMI filter matches `*ubuntu-26.04*` OR `*ubuntu-24.04*` (AWS values=
  array is ORed).

Companion changes for mulga (bootstrap scripts, baremetal, iso pipeline)
land in scripts/tofu-cluster/* and scripts/{tofu-iso,baremetal,iso-builder}/*
under bead mulga-siv-114. Drop the 24.04 fallbacks once all envs are on the
v6 gold image and 24.04 entries are removed from spinifex/utils/images.go.

.github/workflows/e2e-baremetal.yml

@@ -81,14 +81,14 @@ jobs:
 
       - name: Cache VM images
         run: |
-          UBUNTU_IMG="${BM_ISO_CACHE}/ubuntu-24.04-cloudimg-amd64.img"
+          UBUNTU_IMG="${BM_ISO_CACHE}/ubuntu-26.04-cloudimg-amd64.img"
           if [ ! -f "$UBUNTU_IMG" ]; then
-            echo "Downloading Ubuntu 24.04 cloud image..."
+            echo "Downloading Ubuntu 26.04 cloud image..."
             sudo curl -fsSL -o "$UBUNTU_IMG" \
-              "https://cloud-images.ubuntu.com/noble/current/noble-server-cloudimg-amd64.img"
+              "https://cloud-images.ubuntu.com/resolute/current/resolute-server-cloudimg-amd64.img"
             echo "  Downloaded $(du -h "$UBUNTU_IMG" | cut -f1)"
           else
-            echo "Ubuntu 24.04 cloud image cached ($(du -h "$UBUNTU_IMG" | cut -f1))"
+            echo "Ubuntu 26.04 cloud image cached ($(du -h "$UBUNTU_IMG" | cut -f1))"
           fi
 
           LB_IMG="${BM_ISO_CACHE}/lb-alpine-3.21.6-x86_64.raw"
@@ -106,7 +106,7 @@ jobs:
           BM_CI_PASSWORD: ${{ secrets.BM_CI_PASSWORD }}
           BM_SSH_KEY: ${{ runner.temp }}/bm-ci
           BM_EPDU_ENV: ${{ runner.temp }}/epdu.env
-          BM_UBUNTU_IMAGE: ${{ env.BM_ISO_CACHE }}/ubuntu-24.04-cloudimg-amd64.img
+          BM_UBUNTU_IMAGE: ${{ env.BM_ISO_CACHE }}/ubuntu-26.04-cloudimg-amd64.img
           BM_LB_ALPINE_IMAGE: ${{ env.BM_ISO_CACHE }}/lb-alpine-3.21.6-x86_64.raw
           SPX_EXTERNAL_POOL: ${{ env.SPINIFEX_CI_POOL }}
         run: |