[vm-repair] Fix unlock failure on Ubuntu 24.04 ADE-encrypted VMs

The root partition detection in data_os_lvm_check uses a 600MB size
threshold to filter partitions. Ubuntu 24.04 has a ~913MB /boot
partition (partition 16) that also exceeds this threshold, causing
root_part to capture two partitions instead of one.

This results in cryptsetup receiving an invalid device name argument:
  cryptsetup luksOpen ... /dev/sdb1 /dev/sdb16 osencrypt
instead of:
  cryptsetup luksOpen ... /dev/sdb1 osencrypt

The error manifests as:
  Device sdb16 not found
  Cannot use device /dev/sdb16, name is invalid or still in use.

Fix: Replace the fixed-threshold filter with a sort-by-size approach
that selects only the largest partition (which is always the root
partition). This is future-proof against /boot partition size changes.

Additional improvements per review feedback:
- Use $() instead of backticks for command substitution
- Redirect stderr to logfile instead of capturing into the variable
- Quote variables to prevent word-splitting issues
- Separate export from assignment for clarity

Tested on Ubuntu 24.04 Gen 1 and Gen 2 with ADE encryption - unlock
now succeeds. Also verified no regression on Ubuntu 20.04 and 22.04.

Author: msaenzbosupport

src/vm-repair/azext_vm_repair/scripts/linux-mount-encrypted-disk.sh

@@ -95,7 +95,11 @@ data_os_lvm_check () {
 	then
 		#Updaing the below command to use lsblk instead of fdisk for accounting for different distros
 		#export root_part=`fdisk -l ${data_disk} 2>&1 | grep ^/ |awk '$4 > 60000000{print $1}'` >> ${logpath}/${logfile}
-		export root_part=`lsblk ${data_disk} -l -n -p -b 2>&1 | grep -w -v ${data_disk} |awk '$4 > 600000000{print $1}'` >> ${logpath}/${logfile}
+		# Select the largest partition on the data disk (root is always the largest).
+		# Using sort+head instead of a size threshold to avoid matching /boot partitions
+		# that exceed 600MB (e.g. Ubuntu 24.04 has a ~913MB /boot on partition 16).
+		root_part=$(lsblk "${data_disk}" -l -n -p -b 2>>"${logpath}/${logfile}" | grep -w -v "${data_disk}" | sort -k4 -rn | awk 'NR==1{print $1}')
+		export root_part
 		echo "`date` LVM not found on the data disk" >> ${logpath}/${logfile}
 		echo "`date` The OS partition on the data drive is ${root_part}" >> ${logpath}/${logfile}
 	else