fix(compute): adjust default disk size to template

[natalie-o-perret]

Description

WIP

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block, and add the Pull Request #number for each bit you add to the CHANGELOG.md)
• Testing

Testing

$ ./bin/exo compute instance create --zone ch-dk-2 \
    --template 774dd37a-287c-40f0-9e66-343e7b9c9bff \
    template-size-autobump-20260727-2

warning: template "Anapaya Appliance v0.41.1-1" requires at least 80 GiB; increasing the default disk size from 50 GiB to 80 GiB
Creating instance "template-size-autobump-20260727-2"... 1m37s

| ID        | e4609242-b32f-49ef-bb40-e3af8d5d5383 |
| Name      | template-size-autobump-20260727-2    |
| Template  | Anapaya Appliance v0.41.1-1          |
| Zone      | ch-dk-2                              |
| Disk Size | 80 GiB                               |
| State     | running                              |

$ ./bin/exo compute instance create --zone ch-dk-2 \
    --disk-size 50 \
    --template 774dd37a-287c-40f0-9e66-343e7b9c9bff \
    template-size-explicit-20260727

error: --disk-size 50 GiB is smaller than the selected template's minimum of 80 GiB; use --disk-size 80 or larger

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Note

AI assistance: implementation, test scaffolding, PR description.

[natalie-o-perret]

[SC-186913]