[BUG] "group" tag omitted when using job-submit --combine

[pity4yeats]

Describe the bug
When using bkr job-submit --combine to combine several jobs into a single job, the "group" tag in the job XMLs to be combined is omitted.

Version-Release number
beaker-client-28.3-3.fc37.noarch
beaker-common-28.3-3.fc37.noarch

To Reproduce
Steps to reproduce the behavior:

1. Prepare 2 job XMLs with the same contents of the following (notice that in the 1st line group="some-group-in-beaker"):

<job retention_tag="120days" group="some-group-in-beaker">
	<whiteboard>example job xml</whiteboard>
	<recipeSet priority="Normal">
		<recipe whiteboard="BaseOS" ks_meta="harness='restraint-rhts beakerlib-redhat' disable_repo_CRB disable_repo_CRB-debuginfo">
			<distroRequires>
				<and>
					<distro_name op="=" value="RHEL-9.0.0"/>
					<distro_variant op="=" value="BaseOS"/>
					<distro_arch op="=" value="x86_64"/>
				</and>
			</distroRequires>
			<hostRequires/>
			<repos/>
			<partitions/>
			<watchdog panic="ignore"/>
			<task name="/distribution/reservesys" role="None">
				<params>
					<param name="RESERVETIME" value="604800"/>
				</params>
				<rpm name="test(/distribution/reservesys)" path="/mnt/tests/distribution/reservesys"/>
			</task>
		</recipe>
	</recipeSet>
</job>

3. Say the 2 files are named a.xml and b.xml
4. bkr job-submit --combine --xml --dry-run a.xml b.xml

Actual behavior
Only retention_tag is kept, but group="some-group-in-beaker" doesn't present in the resulting XML:

<job retention_tag="120days"><whiteboard>example job xml</whiteboard><recipeSet priority="Normal">
		<recipe whiteboard="BaseOS" ks_meta="harness='restraint-rhts beakerlib-redhat' disable_repo_CRB disable_repo_CRB-debuginfo">
...
		</recipe>
	</recipeSet><recipeSet priority="Normal">
		<recipe whiteboard="BaseOS" ks_meta="harness='restraint-rhts beakerlib-redhat' disable_repo_CRB disable_repo_CRB-debuginfo">
...
		</recipe>
	</recipeSet></job>

Expected behavior
group="some-group-in-beaker" doesn't get omitted in the resulting XML, for example:

<job retention_tag="120days" group="some-group-in-beaker"><whiteboard>example job xml</whiteboard><recipeSet priority="Normal">
		<recipe whiteboard="BaseOS" ks_meta="harness='restraint-rhts beakerlib-redhat' disable_repo_CRB disable_repo_CRB-debuginfo">
...
		</recipe>
	</recipeSet><recipeSet priority="Normal">
		<recipe whiteboard="BaseOS" ks_meta="harness='restraint-rhts beakerlib-redhat' disable_repo_CRB disable_repo_CRB-debuginfo">
...
		</recipe>
	</recipeSet></job>

Screenshots

Additional context