fix: increase max message size

Increase the client-side hardcoded max message size to prevent errors if the server
sends larger than expected messages.

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java

@@ -224,7 +224,7 @@ public class GapicSpannerRpc implements SpannerRpc {
       PathTemplate.create("projects/{project}");
   private static final PathTemplate OPERATION_NAME_TEMPLATE =
       PathTemplate.create("{database=projects/*/instances/*/databases/*}/operations/{operation}");
-  private static final int MAX_MESSAGE_SIZE = 100 * 1024 * 1024;
+  private static final int MAX_MESSAGE_SIZE = 256 * 1024 * 1024;
   private static final int MAX_METADATA_SIZE = 32 * 1024; // bytes
   private static final String PROPERTY_TIMEOUT_SECONDS =
       "com.google.cloud.spanner.watchdogTimeoutSeconds";