fix: get_config reads the backup as text, not JSON

/api/core/backup/download/this answers with config.xml itself, but
get_config went through _get(), which calls .json(). That raised on
every real device, the except swallowed it, and "running" came back
empty. The unit tests replaced _get with a lambda, so they never saw it.

_get_text() returns the body as text. A failed read now raises instead
of returning an empty string, so the caller sees why it failed.

Closes #11

napalm_opnsense/opnsense.py

@@ -172,6 +172,22 @@ class OPNsenseDriver(OPNsensePingMixin, FirewallDriver):
         response.raise_for_status()
         return response.json()
 
+    def _get_text(self, path: str) -> str:
+        """Perform a GET request and return the body as text.
+
+        For the few endpoints that answer with a file rather than JSON, such
+        as the configuration backup.
+
+        :raises ConnectionClosedException: if called before :meth:`open`.
+        :raises RequestException: on HTTP-level errors.
+        """
+        if self.session is None:
+            raise ConnectionClosedException("Not connected – call open() first.")
+        url = self.base_url.rstrip("/") + path
+        response = self.session.get(url, timeout=self.timeout)
+        response.raise_for_status()
+        return response.text
+
     def _post(self, path: str, data: dict[str, Any] | None = None) -> dict[str, Any]:
         """Perform a POST request against the OPNsense REST API.
 
@@ -835,22 +851,19 @@ class OPNsenseDriver(OPNsensePingMixin, FirewallDriver):
         the JSON-serialised staged routes when a candidate has been loaded,
         or an empty string otherwise.
 
-        Calls ``GET /api/core/backup/download/this``.
+        Calls ``GET /api/core/backup/download/this``, which answers with
+        ``config.xml`` itself rather than JSON. A failed read raises: an empty
+        ``running`` would claim the device has no configuration and hide why
+        the read failed, e.g. an API key without the backup privilege.
         """
         configs: dict[str, str] = {"running": "", "startup": "", "candidate": ""}
 
         if retrieve in ("all", "running", "startup"):
-            try:
-                response = self._get("/api/core/backup/download/this")
-                xml_text: str = (
-                    response if isinstance(response, str) else str(response)
-                )
-                if retrieve in ("all", "running"):
-                    configs["running"] = xml_text
-                if retrieve in ("all", "startup"):
-                    configs["startup"] = xml_text
-            except Exception as exc:
-                logger.warning("Failed to fetch running/startup config: %s", exc)
+            xml_text = self._get_text("/api/core/backup/download/this")
+            if retrieve in ("all", "running"):
+                configs["running"] = xml_text
+            if retrieve in ("all", "startup"):
+                configs["startup"] = xml_text
 
         if retrieve in ("all", "candidate") and self._candidate_config is not None:
             configs["candidate"] = json.dumps(self._candidate_config, indent=2)