[Backport fc-26.05-dev] raid_md check: ensure dm-raid module is loaded

[platform-pr-manager]

Bot-based backport to fc-26.05-dev, triggered by a label in #2221. Original body:

All hosts executing the check_md_raid check need to ensure that the dm-raid kernel module is loaded, otherwise the required /proc/mdstat pseudo-file is not present.

PL-135206

@flyingcircusio/release-managers

Release process

This change should be backported:

• Created changelog entry using ./changelog.sh
• Add backport release-25.11 label

This change should only reach this branch:

• Created changelog entry using ./changelog.sh
  or
• Add a upgrade node in doc/upgrade.md

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team

• ensure all checks are green
• get a review from a colleague

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.
• Provide warnings in previous platform version and upgrade notes when introducing a breaking change

Security implications

• Security requirements defined? (WHERE)
  • ensure that requisites for enabled checks are available
• Security requirements tested? (EVIDENCE)
  • manually loaded dm-raid on a host with a failing check