fix(#469): broker blank-field GET leaked the echoed key line

extractScalarValue required ' = ' (space-equals-space) and trimRight()'d first,
so a blank field's reply 'key =' (no space after =) failed the match and the
whole 'mqtt.broker.N.field =' line leaked in as the value. Split on the first
'=' and trim instead: blank -> empty, values with '=' preserved. Regression
tests added.

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

lib/connector/observer_config_client.dart

@@ -241,12 +241,17 @@ class ObserverConfigClient {
     }
   }
 
-  /// Extract the value from a scalar reply ("key = value\n" → "value").
-  /// Falls back to the trimmed whole string if there's no " = ".
+  /// Extract the value from a scalar reply (`key = value` -> `value`).
+  ///
+  /// Splits on the FIRST `=` (config keys never contain `=`; values may). A
+  /// blank field replies `key =` with nothing after the `=`, so the old
+  /// ` = ` (space-equals-space) match failed and leaked the whole `key =` line
+  /// as the value (#469). Splitting on `=` and trimming handles blank values
+  /// (-> empty) and values containing `=` correctly.
   static String extractScalarValue(String reply) {
-    final trimmed = reply.replaceAll('\n', '').trimRight();
-    final i = trimmed.indexOf(' = ');
-    return i < 0 ? trimmed : trimmed.substring(i + 3);
+    final line = reply.replaceAll('\n', '');
+    final i = line.indexOf('=');
+    return i < 0 ? line.trim() : line.substring(i + 1).trim();
   }
 
   /// Read a NUL-terminated UTF-8 string from [f] starting at [start].

test/connector/observer_config_client_test.dart

@@ -208,4 +208,32 @@ void main() {
       expect(list, isNull);
     });
   });
+
+  group('extractScalarValue (#469)', () {
+    String v(String reply) => ObserverConfigClient.extractScalarValue(reply);
+
+    test('blank field returns empty, not the echoed key line', () {
+      // The bug: a blank field replies "key =" (no space after =); the old
+      // " = " match failed and leaked the whole line.
+      expect(v('mqtt.broker.1.iata_override ='), '');
+      expect(v('mqtt.broker.1.iata_override =\n'), '');
+      expect(v('mqtt.broker.1.iata_override = '), '');
+    });
+
+    test('normal value extracted', () {
+      expect(
+        v('mqtt.broker.0.url = mqtt://mqtt1.okimesh.org:1883\n'),
+        'mqtt://mqtt1.okimesh.org:1883',
+      );
+      expect(v('mqtt.broker.0.enabled = 1'), '1');
+    });
+
+    test('value containing = is preserved (split on first =)', () {
+      expect(v('mqtt.broker.0.jwt_token = a=b=c'), 'a=b=c');
+    });
+
+    test('no = falls back to the trimmed line', () {
+      expect(v('  bareword  '), 'bareword');
+    });
+  });
 }