Fix PortChannel name matching in verify_attr_change to handle leading spaces (sonic-net#18301)

What is the motivation for this PR?
test_portchannel_interface_tc2_attributes failed with the following error:

        if attr == "mtu":
            output = duthost.shell("show interfaces status | grep -w '^{}' | awk '{{print $4}}'".format(po_name))
    
>           pytest_assert(output['stdout'] == value, "{} attribute {} failed to change to {}".format(po_name, attr, value))
E           Failed: PortChannel102 attribute mtu failed to change to 3324
This is due to cmd "show interfaces status | grep -w '^{}' | awk '{{print $4}}'" failed to filter out the correct PortChannel interface line when the PortChannel name is indented with spaces:

 PortChannel102              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
 PortChannel104              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
 PortChannel106              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
 PortChannel108              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
 PortChannel109              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
PortChannel1010              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
PortChannel1011              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
PortChannel1012              N/A     100G   9100    N/A           N/A           routed      up       up              N/A         N/A
This PR updates the verify_attr_change function in test_portchannel_interface.py to correctly handle leading spaces in the PortChannel name when parsing interface status output.

How did you do it?
Modified the grep pattern from '^{}' to ^[[:space:]]*{} to match interface lines with leading spaces robustly.

How did you verify/test it?
Run generic_config_updater/test_portchannel_interface.py::test_portchannel_interface_tc2_attributes manually on DUT.

generic_config_updater/test_portchannel_interface.py::test_portchannel_interface_tc2_attributes PASSED

Author: Gfrom2016

tests/generic_config_updater/test_portchannel_interface.py

@@ -247,7 +247,12 @@ def verify_attr_change(duthost, po_name, attr, value):
             ...
     """
     if attr == "mtu":
-        output = duthost.shell("show interfaces status | grep -w '^{}' | awk '{{print $4}}'".format(po_name))
+        cmd = (
+            "show interfaces status | "
+            "grep -w '^[[:space:]]*{}' | "
+            "awk '{{print $4}}'"
+        ).format(po_name)
+        output = duthost.shell(cmd)
 
         pytest_assert(output['stdout'] == value, "{} attribute {} failed to change to {}".format(po_name, attr, value))
     elif attr == "min_links":