feat: updateSystemAi + testSystemAi RPCs (v0.5.0)

`HubClient.updateSystemAi(...)` persists a new System-AI config
to ~/.fai/config.yaml AND hot-reloads the live hub state in one
round-trip; returns the resulting SystemAiStatus so the UI
refreshes without a second call.

`HubClient.testSystemAi()` probes the configured provider with
a tiny ping and returns the same error-kind taxonomy as askAi.
Studio's "Test connection" button uses this.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/src/hub_client.dart

@@ -199,6 +199,32 @@ class HubClient {
     return _admin.askAi(pb.AskAiRequest(prompt: prompt));
   }
 
+  /// Persist a new System-AI configuration. Validates +
+  /// writes back to ~/.fai/config.yaml + hot-reloads the live
+  /// hub state in one call. Returns the resulting status so
+  /// the UI can refresh in one round-trip.
+  Future<SystemAiStatusResponse> updateSystemAi({
+    required String provider,
+    required String endpoint,
+    required String model,
+    required String apiKeyEnv,
+    required String privacyMode,
+  }) {
+    return _admin.updateSystemAi(pb.UpdateSystemAiRequest(
+      provider: provider,
+      endpoint: endpoint,
+      model: model,
+      apiKeyEnv: apiKeyEnv,
+      privacyMode: privacyMode,
+    ));
+  }
+
+  /// Probe the configured provider with a tiny ping. Same
+  /// error-kind taxonomy as [askAi].
+  Future<AskAiResponse> testSystemAi() {
+    return _admin.testSystemAi(Empty());
+  }
+
   /// Search the hub's bundled store index. All filters are
   /// optional; an empty query returns the first [limit] entries
   /// the index ships with.