Serialize BigDecimal as JSON string in V2 request bodies

JsonEncoder.BODY_GSON serialized BigDecimal as a JSON number (e.g., 75.25),
but the V2 API expects decimal fields as JSON strings (e.g., "75.25"). V1
doesn't hit this because FormEncoder calls .toString() which naturally
produces a string.

Adds a TypeAdapter<BigDecimal> that writes toPlainString(). All BigDecimal
fields in stripe-java are format: decimal and expect string wire format.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Committed-By-Agent: claude

Author: jar-stripe

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

@@ -3,15 +3,42 @@
 import com.google.gson.FieldNamingPolicy;
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
 import java.io.IOException;
+import java.math.BigDecimal;
 import java.util.HashMap;
 import java.util.Map;
 
 final class JsonEncoder {
+  /**
+   * Serializes BigDecimal as a JSON string (e.g., "25.5") rather than a JSON number (25.5). All
+   * BigDecimal fields in the Stripe API use format: decimal, and the V2 API expects them as strings
+   * on the wire.
+   */
+  private static final TypeAdapter<BigDecimal> BIG_DECIMAL_STRING_ADAPTER =
+      new TypeAdapter<BigDecimal>() {
+        @Override
+        public void write(JsonWriter out, BigDecimal value) throws IOException {
+          if (value == null) {
+            out.nullValue();
+          } else {
+            out.value(value.toPlainString());
+          }
+        }
+
+        @Override
+        public BigDecimal read(JsonReader in) throws IOException {
+          return new BigDecimal(in.nextString());
+        }
+      };
+
   private static final Gson BODY_GSON =
       new GsonBuilder()
           .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
           .serializeNulls()
+          .registerTypeAdapter(BigDecimal.class, BIG_DECIMAL_STRING_ADAPTER)
           .create();
 
   public static HttpContent createHttpContent(Map<String, Object> params) throws IOException {