fix(api): change an existing VLAN membership instead of re-creating it

set_interface() always POSTed to vlans-ports, treating every membership as
new. Moving a port that already carries the VLAN from untagged to tagged is
not a create, though, and the switch says so:

    POST vlans-ports {"vlan_id":10,"port_id":"10","port_mode":"POM_TAGGED_STATIC"}
      -> 400 {"message":"Association exists"}

Only 409 was handled as "already there"; v7 firmware answers 400. The
membership is its own resource named {vlan_id}-{port_id} and takes a PUT:

    PUT vlans-ports/10-10 -> 200

So the exact case anyone hits first failed outright — a port holding VLAN 10
untagged alongside 30/40/50 tagged, with VLAN 10 to become tagged too.

The response body is now carried into both error messages. The ports PUT just
above already did this; here it was dropped, so "Association exists" — which
names the cause outright — never reached the caller. The message read
"vlans-ports POST HTTP 400" and nothing more.

Verified against a 2530-24G-PoEP on REST v7: set_interface("10", trunk
vlan 30), where that membership already exists, now completes and leaves the
port exactly as it was.

napalm_procurve/procurve.py

@@ -919,28 +919,41 @@ class ProcurveDriver(ConfigLifecycleMixin, SwitchDriver):
         mode = config.get("mode")
         if mode == "trunk":
             for vid in config.get("trunk_vlans", []):
-                payload = {
-                    "vlan_id": vid,
-                    "port_id": interface,
-                    "port_mode": "POM_TAGGED_STATIC",
-                }
-                resp = self._api.post("vlans-ports", json=payload)
-                if not resp.ok and resp.status_code != 409:
-                    raise ConnectionException(
-                        f"set_interface({interface}): vlans-ports POST HTTP {resp.status_code}"
-                    )
+                self._api_set_port_vlan(interface, vid, "POM_TAGGED_STATIC")
         elif mode == "access":
             if "access_vlan" in config:
-                payload = {
-                    "vlan_id": config["access_vlan"],
-                    "port_id": interface,
-                    "port_mode": "POM_UNTAGGED",
-                }
-                resp = self._api.post("vlans-ports", json=payload)
-                if not resp.ok and resp.status_code != 409:
-                    raise ConnectionException(
-                        f"set_interface({interface}): vlans-ports POST HTTP {resp.status_code}"
-                    )
+                self._api_set_port_vlan(interface, config["access_vlan"], "POM_UNTAGGED")
+
+    # Statuses the switch uses to say "that association is already there".
+    # v7 firmware answers 400 with {"message":"Association exists"}; others
+    # use the more conventional 409.
+    _ASSOCIATION_EXISTS = (400, 409)
+
+    def _api_set_port_vlan(self, interface: str, vid: int, port_mode: str) -> None:
+        """Put *interface* into VLAN *vid* with *port_mode*.
+
+        Creating the membership and changing an existing one are different
+        operations here. A port that already carries the VLAN — untagged, say,
+        while it is meant to become tagged — cannot be POSTed again: the switch
+        refuses the duplicate. The membership is its own resource, named
+        ``{vlan_id}-{port_id}``, and changing it is a PUT.
+        """
+        payload = {"vlan_id": vid, "port_id": interface, "port_mode": port_mode}
+        resp = self._api.post("vlans-ports", json=payload)
+        if resp.ok:
+            return
+        if resp.status_code not in self._ASSOCIATION_EXISTS:
+            raise ConnectionException(
+                f"set_interface({interface}): vlans-ports POST HTTP {resp.status_code}"
+                f" – {resp.text[:200]}"
+            )
+
+        resp = self._api.put(f"vlans-ports/{vid}-{interface}", json=payload)
+        if not resp.ok:
+            raise ConnectionException(
+                f"set_interface({interface}): vlans-ports/{vid}-{interface} PUT HTTP "
+                f"{resp.status_code} – {resp.text[:200]}"
+            )
 
     def _cli_set_interface(self, interface: str, config: InterfaceConfigDict) -> None:
         mode = config.get("mode")