nmcli: preserve existing bond mode when omitted on existing connections

Remove hardcoded bond mode default from nmcli module and preserve the existing bond mode during updates when mode is not explicitly supplied.

Include regression coverage and a changelog fragment for issue #9201.

Co-Authored-By: GitHub Copilot <copilot@github.com>

plugins/modules/nmcli.py

@@ -120,10 +120,12 @@ options:
       - loopback
   mode:
     description:
-      - This is the type of device or network connection that you wish to create for a bond or bridge.
+      - Bond mode to use for a bond connection.
+      - When creating a new bond, NetworkManager's default mode V(balance-rr) is used if this option is not provided.
+      - When omitted for an existing bond connection, the module preserves the current bond mode.
+      - This option only applies when C(type) is set to C(bond).
     type: str
     choices: [802.3ad, active-backup, balance-alb, balance-rr, balance-tlb, balance-xor, broadcast]
-    default: balance-rr
   transport_mode:
     description:
       - This option sets the connection type of Infiniband IPoIB devices.
@@ -2818,7 +2820,6 @@ def create_module() -> AnsibleModule:
             # Bond Specific vars
             mode=dict(
                 type="str",
-                default="balance-rr",
                 choices=[
                     "802.3ad",
                     "active-backup",