Update v2 array parameter serialization to use indexed format

Change array parameter serialization for v2 endpoints to use indexed
format (e.g., ?include[0]=foo&include[1]=bar) instead of the repeated
parameter format (e.g., ?include=foo&include=bar). This aligns v2
behavior with v1 for consistency.

Changes:
- Modified FormEncoder.java flattenParamsCollection to always use indexed format
- Updated tests to expect indexed format for v2 arrays

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>
Committed-By-Agent: claude

Author: mbroshi-stripe

src/main/java/com/stripe/net/FormEncoder.java

@@ -249,7 +249,8 @@ private static List<KeyValuePair<String, Object>> flattenParamsCollection(
 
     int index = 0;
     for (Object value : collection) {
-      String newPrefix = arraysAsRepeated ? keyPrefix : String.format("%s[%d]", keyPrefix, index);
+      // Always use indexed format for arrays
+      String newPrefix = String.format("%s[%d]", keyPrefix, index);
       flatParams.addAll(flattenParamsValue(value, newPrefix, arraysAsRepeated));
       index += 1;
     }

src/test/java/com/stripe/net/FormEncoderTest.java

@@ -376,17 +376,17 @@ class TestCase {
                         "array", new Object[] {new String[] {"foo", "bar"}, new int[] {1, 2, 3}}),
                     "array[0][0]=foo&array[0][1]=bar&array[1][0]=1&array[1][1]=2&array[1][2]=3"));
 
-            // Array (arrayAsRepeated)
+            // Array (now always uses indexed format)
             add(new TestCase(Collections.singletonMap("array", new String[] {}), "", true));
             add(
                 new TestCase(
                     Collections.singletonMap("array", new String[] {"1", "2", "3"}),
-                    "array=1&array=2&array=3",
+                    "array[0]=1&array[1]=2&array[2]=3",
                     true));
             add(
                 new TestCase(
                     Collections.singletonMap("array", new Object[] {123, "foo"}),
-                    "array=123&array=foo",
+                    "array[0]=123&array[1]=foo",
                     true));
             // Collection
             add(