test(#70): lock observer config model behavior (M1)

The models are the existing draft (1983ed9); Gemini flagged nothing in them.
This pins the defensive behavior they give untrusted wire data: enum fromWire
fallback, numeric tryParse fallback, secret-presence flag, SecretField
three-state, copyWith. 7 tests, all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/models/observer_config_test.dart

@@ -0,0 +1,91 @@
+// Lock tests for the Offband config models (#64 M1).
+//
+// The models are the data structures untrusted wire fields decode into, so the
+// defensive behavior (enum fallback, numeric fallback, secret presence) is
+// pinned here. All green — M1 confirms the existing draft is contract-correct.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/models/observer_config.dart';
+
+void main() {
+  group('enums fromWire / wire', () {
+    test('BrokerTransport round-trips + rejects unknown', () {
+      expect(BrokerTransport.fromWire('tcp'), BrokerTransport.tcp);
+      expect(BrokerTransport.fromWire('tls'), BrokerTransport.tls);
+      expect(BrokerTransport.fromWire('wss'), BrokerTransport.wss);
+      expect(BrokerTransport.fromWire('bogus'), isNull);
+      expect(BrokerTransport.tls.wire, 'tls');
+    });
+
+    test('BrokerAuthType round-trips + rejects unknown', () {
+      expect(BrokerAuthType.fromWire('none'), BrokerAuthType.none);
+      expect(BrokerAuthType.fromWire('jwt'), BrokerAuthType.jwt);
+      expect(BrokerAuthType.fromWire('nope'), isNull);
+      expect(BrokerAuthType.basic.wire, 'basic');
+    });
+
+    test('WifiStatus maps tokens, unknown -> unknown', () {
+      expect(WifiStatus.fromWire('StaConnected'), WifiStatus.staConnected);
+      expect(WifiStatus.fromWire('StaFailed'), WifiStatus.staFailed);
+      expect(WifiStatus.fromWire('weird'), WifiStatus.unknown);
+    });
+  });
+
+  group('BrokerConfig.fromWireFields', () {
+    test('maps a fully-populated slot', () {
+      final b = BrokerConfig.fromWireFields(3, {
+        'enabled': '1',
+        'url': 'mqtt://h',
+        'port': '8883',
+        'transport': 'tls',
+        'auth_type': 'jwt',
+        'username': 'u',
+        'password': '(set)',
+        'topic_prefix': 'p',
+        'jwt_refresh': '900',
+      });
+      expect(b.slot, 3);
+      expect(b.enabled, isTrue);
+      expect(b.url, 'mqtt://h');
+      expect(b.port, 8883);
+      expect(b.transport, BrokerTransport.tls);
+      expect(b.authType, BrokerAuthType.jwt);
+      expect(b.username, 'u');
+      expect(b.passwordSet, isTrue);
+      expect(b.jwtRefresh, 900);
+      expect(b.isPopulated, isTrue);
+    });
+
+    test('defaults + enum fallback on a sparse/garbage slot', () {
+      final b = BrokerConfig.fromWireFields(0, {
+        'transport': 'bogus',
+        'auth_type': '?',
+        'password': '(unset)',
+      });
+      expect(b.url, '');
+      expect(b.port, 0);
+      expect(b.transport, BrokerTransport.tcp); // unknown -> default
+      expect(b.authType, BrokerAuthType.none); // unknown -> default
+      expect(b.passwordSet, isFalse); // not "(set)"
+      expect(b.isPopulated, isFalse); // no url
+    });
+  });
+
+  group('SecretField three-state', () {
+    test('unchanged / clear / set', () {
+      expect(const SecretField.unchanged(), isA<SecretUnchanged>());
+      expect(const SecretField.clear(), isA<SecretClear>());
+      const s = SecretField.set('x');
+      expect(s, isA<SecretSet>());
+      expect((s as SecretSet).value, 'x');
+    });
+  });
+
+  test('BrokerConfig.copyWith overrides selectively', () {
+    const b = BrokerConfig(slot: 1, url: 'a', port: 1);
+    final c = b.copyWith(url: 'b');
+    expect(c.slot, 1);
+    expect(c.url, 'b');
+    expect(c.port, 1); // untouched
+  });
+}