Added missing sync unit tests (#83)

## This PR
Adds missing sync structure unit tests. It mostly tests the validation
of incoming flags, but additionally checks if correct configs are
registered properly (both flags and metadata).


### How to test
```
bazelisk test //providers/flagd/tests:sync_test
```

---------

Signed-off-by: Marcin Olko <molko@google.com>

Author: m-olko

providers/flagd/tests/BUILD

@@ -8,3 +8,13 @@ cc_test(
         "@googletest//:gtest_main",
     ],
 )
+
+cc_test(
+    name = "sync_test",
+    srcs = ["sync_test.cpp"],
+    deps = [
+        "//providers/flagd/src/sync",
+        "@googletest//:gtest_main",
+        "@nlohmann_json//:json",
+    ],
+)

providers/flagd/tests/sync_test.cpp

@@ -0,0 +1,132 @@
+#include "flagd/sync/sync.h"
+
+#include <gtest/gtest.h>
+
+#include <nlohmann/json.hpp>
+
+namespace flagd {
+
+class TestableSync : public FlagSync {
+ public:
+  using FlagSync::FlagSync;
+
+  absl::Status Init(const openfeature::EvaluationContext& ctx) override {
+    return absl::OkStatus();
+  }
+  absl::Status Shutdown() override { return absl::OkStatus(); }
+
+  void TriggerUpdate(const nlohmann::json& new_json) {
+    this->UpdateFlags(new_json);
+  }
+};
+
+class SyncTest : public ::testing::Test {
+ protected:
+  TestableSync sync_;
+};
+
+TEST_F(SyncTest, ValidatorAcceptsValidJson) {
+  nlohmann::json valid_json = R"({
+    "flags": {
+      "my-flag": {
+        "state": "ENABLED",
+        "variants": {
+          "on": true,
+          "off": false
+        },
+        "defaultVariant": "on"
+      }
+    }
+  })"_json;
+
+  sync_.TriggerUpdate(valid_json);
+
+  std::shared_ptr<const nlohmann::json> flags = sync_.GetFlags();
+  EXPECT_TRUE(flags->contains("my-flag"));
+}
+
+TEST_F(SyncTest, ValidatorRejectsInvalidJson) {
+  // Missing 'flags' field
+  nlohmann::json invalid_json = R"({
+    "something": "else"
+  })"_json;
+
+  sync_.TriggerUpdate(invalid_json);
+
+  std::shared_ptr<const nlohmann::json> flags = sync_.GetFlags();
+  EXPECT_TRUE(flags->empty());
+}
+
+TEST_F(SyncTest, ValidatorRejectsInvalidType) {
+  // 'state' should be a string, not an integer
+  nlohmann::json invalid_json = R"({
+    "flags": {
+      "my-flag": {
+        "state": 123,
+        "variants": {
+          "on": true,
+          "off": false
+        },
+        "defaultVariant": "on"
+      }
+    }
+  })"_json;
+
+  sync_.TriggerUpdate(invalid_json);
+
+  std::shared_ptr<const nlohmann::json> flags = sync_.GetFlags();
+  EXPECT_TRUE(flags->empty());
+}
+
+TEST_F(SyncTest, ValidatorRejectsMissingVariants) {
+  // Missing 'variants' field
+  nlohmann::json invalid_json = R"({
+    "flags": {
+      "my-flag": {
+        "state": "ENABLED",
+        "defaultVariant": "on"
+      }
+    }
+  })"_json;
+
+  sync_.TriggerUpdate(invalid_json);
+
+  std::shared_ptr<const nlohmann::json> flags = sync_.GetFlags();
+  EXPECT_TRUE(flags->empty());
+}
+
+TEST_F(SyncTest, ValidatorRejectsMalformedFlag) {
+  // 'state' has invalid value
+  nlohmann::json invalid_json = R"({
+    "flags": {
+      "my-flag": {
+        "state": "INVALID_STATE",
+        "variants": {
+          "on": true
+        },
+        "defaultVariant": "on"
+      }
+    }
+  })"_json;
+
+  sync_.TriggerUpdate(invalid_json);
+
+  std::shared_ptr<const nlohmann::json> flags = sync_.GetFlags();
+  EXPECT_TRUE(flags->empty());
+}
+
+TEST_F(SyncTest, MetadataIsExtracted) {
+  nlohmann::json json_with_metadata = R"({
+    "flags": {},
+    "metadata": {
+      "foo": "bar"
+    }
+  })"_json;
+
+  sync_.TriggerUpdate(json_with_metadata);
+
+  std::shared_ptr<const nlohmann::json> metadata = sync_.GetMetadata();
+  EXPECT_EQ((*metadata)["foo"], "bar");
+}
+
+}  // namespace flagd