fix(agent): adding checks for session and explicit systemd requirement (#166)

[Dimfacion]

Proposed changes

• adding checks for session and explicit systemd requirement

Testing Instructions

1. On linux arm64, try to install the agent for all the different mode
2. Check they are all working or explicitly explain the issue

Related issues

• Closes fix: systemctl degraded for Linux agent #166

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality
• For bug fix -> I implemented a test that covers the bug

[Copilot]

Pull request overview

This PR improves the Linux agent installer scripts’ handling of systemd-related prerequisites so installations fail with clearer messaging (and don’t abort silently under set -e when systemctl is-system-running returns a non-zero exit code such as degraded).

Changes:

• Make systemctl is-system-running non-fatal under set -e by suppressing stderr and forcing success.
• Add explicit prerequisite messaging for service/service-user modes when systemd is not in an acceptable state.
• Add a session-user guard to ensure the systemd user manager is available before proceeding.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
installer/linux/agent-installer.sh	Makes systemd state detection non-fatal and adds a prerequisite message for service installs.
installer/linux/agent-installer-session-user.sh	Makes systemd state detection non-fatal and adds a check for a usable systemd user manager in the session-user flow.
installer/linux/agent-installer-service-user.sh	Makes systemd state detection non-fatal and adds a prerequisite message for service-user installs.

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[savacano28]

If systemd_status = true, the message is:
echo "Systemd is in unexpected state: $(true). Installation is not supported." (same comment as Copilot)

[savacano28]

Code is OK, just a small fix related to the display message.