[App Check] Fix JSON key ordering in HeartbeatsPayload (#11482)

Key ordering from `JSONEncoder` is not guaranteed, resulting in test failures when output is compared directly.

Added the `sortedKeys` output formatting option in debug builds to fix non-deterministic test results while avoiding unnecessary sorting costs in release builds.

Author: andrewheard

FirebaseCore/Internal/Sources/HeartbeatLogging/HeartbeatsPayload.swift

@@ -91,6 +91,14 @@ extension HeartbeatsPayload: HTTPHeaderRepresentable {
   public func headerValue() -> String {
     let encoder = JSONEncoder()
     encoder.dateEncodingStrategy = .formatted(Self.dateFormatter)
+    #if DEBUG
+      // TODO: Remove the following #available check when FirebaseCore's minimum deployment target
+      // is iOS 11+; all other supported platforms already meet the minimum for `.sortedKeys`.
+      if #available(iOS 11, *) {
+        // Sort keys in debug builds to simplify output comparisons in unit tests.
+        encoder.outputFormatting = .sortedKeys
+      }
+    #endif // DEBUG
 
     guard let data = try? encoder.encode(self) else {
       // If encoding fails, fall back to encoding with an empty payload.