Add VLAN range expansion utility and corresponding unit tests

- Introduced a new utility function `expand_vlan_range` in `utils.py` to expand VLAN range strings into individual VLAN IDs.
- Created a new test file `test_units.py` with unit tests for the `expand_vlan_range` function, covering various scenarios including simple ranges, reversed ranges, non-numeric inputs, and list inputs.
- Enhanced test coverage for other functionalities in the `device_registry` and template validation classes.

oxi/interfaces/utils.py

@@ -0,0 +1,25 @@
+def expand_vlan_range(value: str | list[str]) -> list[str]:
+    """Expand values like '1,7,14-15' into individual VLAN IDs."""
+    if isinstance(value, list):
+        value = ",".join(str(item) for item in value)
+
+    result: list[str] = []
+    if not value:
+        return result
+    for part in value.split(","):
+        part = part.strip()
+        if not part:
+            continue
+        if "-" in part:
+            start_s, end_s = part.split("-", 1)
+            try:
+                start, end = int(start_s), int(end_s)
+            except ValueError:
+                result.append(part)
+                continue
+            if start > end:
+                start, end = end, start
+            result.extend(str(i) for i in range(start, end + 1))
+        else:
+            result.append(part)
+    return result