refactor: change header handling to support immutable maps

Author: ewanharris

src/main/java/dev/openfga/sdk/api/client/OpenFgaClient.java

@@ -838,12 +838,13 @@ public CompletableFuture<List<ClientBatchCheckClientResponse>> clientBatchCheck(
         var options = batchCheckOptions != null
                 ? batchCheckOptions
                 : new ClientBatchCheckClientOptions().maxParallelRequests(DEFAULT_MAX_METHOD_PARALLEL_REQS);
-        if (options.getAdditionalHeaders() == null) {
-            options.additionalHeaders(new HashMap<>());
-        }
-        options.getAdditionalHeaders().putIfAbsent(CLIENT_METHOD_HEADER, "ClientBatchCheck");
-        options.getAdditionalHeaders()
-                .putIfAbsent(CLIENT_BULK_REQUEST_ID_HEADER, randomUUID().toString());
+
+        HashMap<String, String> headers = options.getAdditionalHeaders() != null
+                ? new HashMap<>(options.getAdditionalHeaders())
+                : new HashMap<>();
+        headers.putIfAbsent(CLIENT_METHOD_HEADER, "ClientBatchCheck");
+        headers.putIfAbsent(CLIENT_BULK_REQUEST_ID_HEADER, randomUUID().toString());
+        options.additionalHeaders(headers);
 
         int maxParallelRequests = options.getMaxParallelRequests() != null
                 ? options.getMaxParallelRequests()
@@ -1130,12 +1131,13 @@ public CompletableFuture<ClientListRelationsResponse> listRelations(
         var options = listRelationsOptions != null
                 ? listRelationsOptions
                 : new ClientListRelationsOptions().maxParallelRequests(DEFAULT_MAX_METHOD_PARALLEL_REQS);
-        if (options.getAdditionalHeaders() == null) {
-            options.additionalHeaders(new HashMap<>());
-        }
-        options.getAdditionalHeaders().putIfAbsent(CLIENT_METHOD_HEADER, "ListRelations");
-        options.getAdditionalHeaders()
-                .putIfAbsent(CLIENT_BULK_REQUEST_ID_HEADER, randomUUID().toString());
+
+        HashMap<String, String> headers = options.getAdditionalHeaders() != null
+                ? new HashMap<>(options.getAdditionalHeaders())
+                : new HashMap<>();
+        headers.put(CLIENT_METHOD_HEADER, "ListRelations");
+        headers.put(CLIENT_BULK_REQUEST_ID_HEADER, randomUUID().toString());
+        options.additionalHeaders(headers);
 
         var batchCheckRequests = request.getRelations().stream()
                 .map(relation -> new ClientCheckRequest()

src/test/java/dev/openfga/sdk/api/client/OpenFgaClientHeadersTest.java

@@ -516,9 +516,8 @@ public void clientBatchCheck_withHeaders() throws Exception {
                 ._object(DEFAULT_OBJECT)
                 .relation(DEFAULT_RELATION)
                 .user(DEFAULT_USER);
-        Map<String, String> headers = new java.util.HashMap<>();
-        headers.put("test-header", "test-value-per-call");
-        ClientBatchCheckClientOptions options = new ClientBatchCheckClientOptions().additionalHeaders(headers);
+        ClientBatchCheckClientOptions options =
+                new ClientBatchCheckClientOptions().additionalHeaders(Map.of("test-header", "test-value-per-call"));
 
         // When
         List<ClientBatchCheckClientResponse> response =
@@ -748,10 +747,8 @@ public void listRelations_withHeaders() throws Exception {
                 .relations(List.of(DEFAULT_RELATION))
                 .user(DEFAULT_USER)
                 ._object(DEFAULT_OBJECT);
-        // Use HashMap instead of Map.of() to create a mutable map
-        Map<String, String> headers = new java.util.HashMap<>();
-        headers.put("test-header", "test-value-per-call");
-        ClientListRelationsOptions options = new ClientListRelationsOptions().additionalHeaders(headers);
+        ClientListRelationsOptions options =
+                new ClientListRelationsOptions().additionalHeaders(Map.of("test-header", "test-value-per-call"));
 
         // When
         ClientListRelationsResponse response =