Crash when attempting to instantiate via_stack with metal_level==2

[jreeve-nl]

Describe the bug
In the existing code for handling a via_stack than spans metal2 or above, a subscriptable type is expected of m_enc which is the overhang of the metal compared to the contact.

To Reproduce

if __name__ == "__main__":
    
    PDK.activate()
    c = gf.Component()
    nfet_lv_m6 = c << gf180mcu.cells.via_stack(
        x_range=(0, 1),
        y_range=(0, 1),
        base_layer=PDK.get_layer("comp"),
        metal_level=2,
        li_enc_dir="H",
    )

    gdspath = c.write_gds()
    gf.show(gdspath)

Traceback (most recent call last):
  File "/home/jsr/dev/kiwi-devices/gf-fundamental-blocks/gdsfactory/./5t_ota_simple.py", line 131, in <module>
    nfet_lv_m6 = c << gf180mcu.cells.via_stack(
                      ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsr/dev-ext/gdsfactory/gdsfactory/.venv/lib/python3.12/site-packages/kfactory/layout.py", line 1029, in func
    return wrapper_autocell(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsr/dev-ext/gdsfactory/gdsfactory/.venv/lib/python3.12/site-packages/kfactory/decorators.py", line 549, in __call__
    return self._f(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsr/dev-ext/gdsfactory/gdsfactory/.venv/lib/python3.12/site-packages/kfactory/decorators.py", line 523, in wrapper_autocell
    cell_ = wrapped_cell(**params)
            ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsr/dev-ext/gdsfactory/gdsfactory/.venv/lib/python3.12/site-packages/cachetools/_cached.py", line 173, in wrapper
    v = func(*args, **kwargs)
        ^^^^^^^^^^^^^^^^^^^^^
  File "/home/jsr/dev-ext/gdsfactory/gdsfactory/.venv/lib/python3.12/site-packages/kfactory/decorators.py", line 408, in wrapped_cell
    cell = f(**params)  # type: ignore[call-arg]
           ^^^^^^^^^^^
  File "/home/jsr/dev-ext/gdsfactory/gf180mcu-kiwi-devices/gf180mcu/cells/via_generator.py", line 153, in via_stack
    if (via1.dxmax - via1.dxmin + 2 * m_enc[0]) < (
                                      ~~~~~^^^
TypeError: 'float' object is not subscriptable

Expected behavior
Draw the via stack with output on metal layers 1 and 2.

Suggested fix
I have created a PR that alters the via_stack argument type of m_enc to be a tuple containing the x and y overhang of the contact.

Environment (please complete the following information):

• [ O] I have reviewed the documentation, discussions and issues and found no relevant solution.
• [ O] I understand that this is an open-source project, and maintainers may not have the resources to address every issue. I am prepared to contribute by fixing the issue myself or hiring someone to do so, if needed, and accept that the issue may not be resolved otherwise.
• [ O

] I am using the latest version of GDSFactory, with Python 3.11, 3.12, or 3.13.

Please provide the output for the following code:

import sys
print(sys.version)
print(sys.executable)

import gdsfactory as gf
gf.config.print_version_plugins()