[MM-60059] Point guest-only inviters to an administrator when the invitee is already a workspace member (#38558)

* [MM-60059] Direct guest-only inviters to an admin when the invitee is already a member

When a non-guest is selected in the guest invite flow, the invite modal told
every inviter to "invite them as a member instead", even when the inviter has
invite_guest but not add_user_to_team and therefore has no member/guest toggle
to switch to. Pick the message based on the inviter's add_user_to_team
permission on the target team.

Co-authored-by: mattermost-code <matty-code@mattermost.com>

* Cover the guest-invite permission check with mutation-tested cases

Mutation testing showed the permitted-inviter case was only ever proven through
a system role, so dropping the team id argument from haveITeamPermission left
every test green. Cover an inviter whose add_user_to_team comes from a team
role, an inviter with no membership in the invited team, and a batch holding
more than one non-guest so the flag cannot be applied to only the first
invitee.

Co-authored-by: mattermost-code <matty-code@mattermost.com>

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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: mattermost-code <matty-code@mattermost.com>