feat(middleware): add MQTT payload parser with dot-notation JSON path extraction

- src/modules/mqtt/payload-parser.ts: extractValue() supporting \$.field and \$.nested.deep paths
- Returns raw string when no path provided, null for invalid JSON or missing paths
- Stringifies numbers/booleans, returns JSON string for objects/arrays
- tests/payload-parser.test.ts: 19 tests covering no-path passthrough, simple/nested fields, missing paths, invalid JSON, and edge cases (null, zero, arrays)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

display/middleware/src/modules/mqtt/payload-parser.ts

@@ -0,0 +1,62 @@
+/**
+ * Extract a value from a JSON payload using a simple dot-notation JSON path.
+ *
+ * Supported path format: "$.field" or "$.field.nested.deep"
+ * - Returns the raw payload string if no path is provided (undefined, null, or empty string).
+ * - Returns null if the JSON is invalid or the path does not exist.
+ * - Returns the extracted value as a string (numbers, booleans, null are stringified).
+ * - Objects and arrays are returned as their JSON string representation.
+ */
+export function extractValue(
+  payload: string,
+  jsonPath: string | null | undefined
+): string | null {
+  // No path — return raw payload as-is
+  if (!jsonPath) {
+    return payload;
+  }
+
+  // Parse JSON
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(payload);
+  } catch {
+    return null;
+  }
+
+  // Remove leading "$." and split on dots
+  const pathWithoutRoot = jsonPath.startsWith('$.') ? jsonPath.slice(2) : jsonPath;
+
+  if (!pathWithoutRoot) {
+    // Path was just "$" — return the whole object as JSON
+    return JSON.stringify(parsed);
+  }
+
+  const segments = pathWithoutRoot.split('.');
+
+  // Traverse the parsed object
+  let current: unknown = parsed;
+  for (const segment of segments) {
+    if (current === null || typeof current !== 'object' || Array.isArray(current)) {
+      return null;
+    }
+    const obj = current as Record<string, unknown>;
+    if (!Object.prototype.hasOwnProperty.call(obj, segment)) {
+      return null;
+    }
+    current = obj[segment];
+  }
+
+  // Convert result to string
+  if (current === undefined) {
+    return null;
+  }
+  if (current === null) {
+    return 'null';
+  }
+  if (typeof current === 'object') {
+    // Arrays and objects: return JSON string without spaces
+    return JSON.stringify(current);
+  }
+  return String(current);
+}