Runtime error raised by get_interfaces_by_mac_on_linux() when multiple gre tunnels are configured

# Bug report
The get_interfaces_by_mac_on_linux() function lists the hex conversion from the source ip address as mac address for a gre tunnel, and raises a runtime error when multiple gre tunnels have the same source ip address.

## Steps to reproduce the problem
1. Use a cloud provider that calls net.get_interfaces_by_mac() in their hotplug script
2. Configure multiple gre tunnels with the same source ip address

## Environment details
- Cloud-init version: 26.1-0ubuntu2
- Operating System Distribution: Ubuntu 26.04 LTS
- Cloud provider, platform or installer type: Hetzner

## cloud-init logs
```
hotplug_hook.py[ERROR]:**` Received fatal exception handling hotplug!

Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/cloudinit/cmd/devel/hotplug_hook.py", line 327, in handle_args
handle_hotplug(
~~~~~~~~~~~~~~^
hotplug_init=hotplug_init,
^^^^^^^^^^^^^^^^^^^^^^^^^^
...<2 lines>...
udevaction=args.udevaction,
^^^^^^^^^^^^^^^^^^^^^^^^^^^
--
private_network["mac_address"]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
),
^
File "/usr/lib/python3/dist-packages/cloudinit/sources/helpers/hetzner.py", line 42, in get_interface_name_from_mac
mac_to_iface = net.get_interfaces_by_mac()
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 898, in get_interfaces_by_mac
return get_interfaces_by_mac_on_linux()
File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 997, in get_interfaces_by_mac_on_linux
raise RuntimeError(msg)
RuntimeError: duplicate mac found! both 'gre_xx' and 'gre_yy' have mac 'xx:xx:xx:xx'.
```
## temporary workaround
As gre tunnels have not set driver or _devid, we can extend the test short-term fix of LP: #1997922 and replace

> if driver in ("fsl_enetc", "mscc_felix", "qmi_wwan"):

by
> if (driver is None and _devid is None) or driver in ("fsl_enetc", "mscc_felix", "qmi_wwan"): >

This can be done using a single sed command:
```
sed -ie '/if driver in ("fsl_enetc", "mscc_felix", "qmi_wwan"):/s/if/if (driver is None and _devid is None) or/' /usr/lib/python3/dist-packages/cloudinit/net/__init__.py
```

Start in cloudinit/net/__init__.py at get_interfaces_by_mac_on_linux() and inspect the duplicate-MAC handling shown in the traceback. Reproduce the case with multiple GRE tunnels sharing a source IP, then verify that get_interfaces_by_mac() no longer raises a runtime error for those interfaces while preserving duplicate detection for real conflicts.

Written by the indexing model from the issue text.