feat: Add tests for implicit identity key (#34)

Author: khvn26

test_cases/test_multivariate__explicit_identity_key__expected_allocation.jsonc

@@ -0,0 +1,43 @@
+{
+  // Given: A multivariate feature with one variant and an identity key resulting in %54.53 allocation
+  // When: Evaluating the feature for the given identity
+  // Then: The variant should be selected as the feature value
+  "$schema": "https://raw.githubusercontent.com/Flagsmith/engine-test-data/refs/tags/v2.0.0/schema.json",
+  "context": {
+    "environment": {
+      "key": "test_env",
+      "name": "Test Environment"
+    },
+    "identity": {
+      "identifier": "foobar",
+      "key": "test_env_foobar"
+    },
+    "features": {
+      "multivariate_feature": {
+        "enabled": true,
+        "key": "78986",
+        "name": "multivariate_feature",
+        "value": "foo",
+        "variants": [
+          {
+            "value": "variant_value",
+            "weight": 54.54,
+            "priority": 1
+          }
+        ]
+      }
+    },
+    "segments": {}
+  },
+  "result": {
+    "flags": {
+      "multivariate_feature": {
+        "enabled": true,
+        "name": "multivariate_feature",
+        "reason": "SPLIT; weight=54.54",
+        "value": "variant_value"
+      }
+    },
+    "segments": []
+  }
+}

test_cases/test_multivariate__no_identity_key__expected_allocation.jsonc

@@ -0,0 +1,47 @@
+{
+  // Given: A multivariate feature with one variant and an identity identifier
+  //        expected to produce an identity key resulting in %54.53 allocation
+  // When: Evaluating the feature for the given identity
+  // Then: The variant should be selected as the feature value.
+  //
+  // NOTE: The expectation is that the engine constructs the identity key internally
+  //       using the environment key and identity identifier. In this case, we expect
+  //       the constructed key to be "test_env_foobar" which results in 54.53% allocation.
+  "$schema": "https://raw.githubusercontent.com/Flagsmith/engine-test-data/refs/tags/v2.0.0/schema.json",
+  "context": {
+    "environment": {
+      "key": "test_env",
+      "name": "Test Environment"
+    },
+    "identity": {
+      "identifier": "foobar"
+    },
+    "features": {
+      "multivariate_feature": {
+        "enabled": true,
+        "key": "78986",
+        "name": "multivariate_feature",
+        "value": "foo",
+        "variants": [
+          {
+            "value": "variant_value",
+            "weight": 54.54,
+            "priority": 1
+          }
+        ]
+      }
+    },
+    "segments": {}
+  },
+  "result": {
+    "flags": {
+      "multivariate_feature": {
+        "enabled": true,
+        "name": "multivariate_feature",
+        "reason": "SPLIT; weight=54.54",
+        "value": "variant_value"
+      }
+    },
+    "segments": []
+  }
+}