Check if eMMC is mounted before writing during multipart update.

This change adds a check to ensure that the eMMC filesystem is mounted before attempting to write firmware bundle files during a multipart update. If the eMMC is not mounted, an error is returned, preventing writes to an unexpected location. The return type of CreateBoostRequest is also updated to StatusOr to propagate errors.

I choose sys/stat to avoid DBus dependencies (which is normally async).

This will avoid all future problem that firmware bundle is placed into rwfs and brick BMC, because of eMMC not being ready.

#bmc-bloom

No eMMC.
```
root@ddcbs3-nfd01:~# umount /mnt/luks-mmcblk0_fs/
root@ddcbs3-nfd01:~# df -h
Filesystem                Size      Used Available Use% Mounted on
dev                     468.4M         0    468.4M   0% /dev
tmpfs                   469.4M     13.3M    456.1M   3% /run
/dev/mtdblock4           52.1M     52.1M         0 100% /run/initramfs/ro
/dev/mtdblock7            3.0M    956.0K      2.1M  31% /run/initramfs/rw
cow                       3.0M    956.0K      2.1M  31% /
tmpfs                   469.4M      8.0K    469.4M   0% /dev/shm
tmpfs                   469.4M      8.7M    460.7M   2% /tmp
tmpfs                   469.4M     52.0K    469.3M   0% /var/volatile
tmpfs                   469.4M     13.3M    456.1M   3% /var/lib/systemd/coredump
root@ddcbs3-nfd01:~# mkdir /mnt/luks-mmcblk0_fs/
mkdir: can't create directory '/mnt/luks-mmcblk0_fs/': File exists
root@ddcbs3-nfd01:~# touch /mnt/luks-mmcblk0_fs/something.txt
root@ddcbs3-nfd01:~# echo "hello" >> /mnt/luks-mmcblk0_fs/something.txt
root@ddcbs3-nfd01:~# cat /mnt/luks-mmcblk0_fs/something.txt
hello

ddcbs3-n1:~# /tmp/platforms_milotic_milotictool_mpm/1-23c4ce84_050585c7_e0f8d895_e258b4ad_27400fc6/milotic_tool_cli  -uid= -gid=   --auth insecure --command post --host ddcbs3-nfd01 --port 3995 --resource_url /redfish/v1/UpdateService/MultipartUpdate --command_body_file firmware_bundle.tar.gz
E0220 17:57:13.988465   99321 google_no_envelope_resolver.cc:195] UNAVAILABLE: (ddcbs3-nfd01:3995): unknown resolver mechanism. For DNS resolution, please prepend addresses with 'dns:///'.
=== Source Location Trace: ===
net/grpc/util/address_updater/address_updater.cc:951
URL is /redfish/v1/UpdateService/MultipartUpdate
Command body will be parsed from firmware_bundle.tar.gz
F0220 17:57:14.040414   99229 grpc_dynamic_raw.cc:382] Command failed: INTERNAL: eMMC is not mounted. Can't process multipart update.
=== Source Location Trace: ===
third_party/ecclesia/lib/status/rpc.h:45

Body is
```

Have eMMC

```
root@ddcbs3-nfd01:~# df -h
Filesystem                Size      Used Available Use% Mounted on
dev                     468.4M         0    468.4M   0% /dev
tmpfs                   469.4M      4.2M    465.2M   1% /run
/dev/mtdblock4           52.1M     52.1M         0 100% /run/initramfs/ro
/dev/mtdblock7            3.0M    956.0K      2.1M  31% /run/initramfs/rw
cow                       3.0M    956.0K      2.1M  31% /
tmpfs                   469.4M      8.0K    469.4M   0% /dev/shm
tmpfs                   469.4M    224.0K    469.2M   0% /tmp
tmpfs                   469.4M     52.0K    469.3M   0% /var/volatile
tmpfs                    14.2G    304.2M     13.2G   2% /var/lib/systemd/coredump
/dev/mapper/luks-mmcblk0
                         14.2G    304.2M     13.2G   2% /mnt/luks-mmcblk0_fs
/dev/mapper/luks-mmcblk0
                         14.2G    304.2M     13.2G   2% /run/systemd/coredumps
/dev/mapper/luks-mmcblk0
                         14.2G    304.2M     13.2G   2% /var/lib/systemd/coredump

ddcbs3-n1:~# /tmp/platforms_milotic_milotictool_mpm/1-23c4ce84_050585c7_e0f8d895_e258b4ad_27400fc6/milotic_tool_cli  -uid= -gid=   --auth insecure --command post --host ddcbs3-nfd01 --port 3995 --resource_url /redfish/v1/UpdateService/MultipartUpdate --command_body_file firmware_bundle.tar.gz
E0220 18:09:52.075019  103457 google_no_envelope_resolver.cc:195] UNAVAILABLE: (ddcbs3-nfd01:3995): unknown resolver mechanism. For DNS resolution, please prepend addresses with 'dns:///'.
=== Source Location Trace: ===
net/grpc/util/address_updater/address_updater.cc:951
URL is /redfish/v1/UpdateService/MultipartUpdate
Command body will be parsed from firmware_bundle.tar.gz
Response JSON is /redfish/v1/UpdateService/MultipartUpdate{
  "@odata.id": "/redfish/v1/TaskService/Tasks/FirmwareBundleUpdate",
  "@odata.type": "#Task.v1_4_2.Task",
  "Id": "FirmwareBundleUpdate",
  "Name": "Firmware Bundle Update",
  "PercentComplete": 0,
  "TaskState": "New"
}
Response JSON is /redfish/v1/UpdateService/MultipartUpdate{
  "@odata.id": "/redfish/v1/TaskService/Tasks/FirmwareBundleUpdate",
  "@odata.type": "#Task.v1_4_2.Task",
  "Id": "FirmwareBundleUpdate",
  "Name": "Firmware Bundle Update",
  "PercentComplete": 0,
  "TaskState": "New"
}

```

PiperOrigin-RevId: 873161540
Change-Id: Ibe3dc55062a5be62f91425c2ff33533883559b7d