Skip Jackson serialization for primitives in PyishObjectMapper

In 3.0, LegacyOverrides.THREE_POINT_0 defaults usePyishObjectMapper=true,
causing every expression output to go through full Jackson ObjectWriter
serialization. For String/Number/Boolean (the vast majority of expression
results), this is unnecessary -- val.toString() produces the same output.

This was the single largest source of the 3.0 vs 2.8.x regression.

Benchmark: +104% throughput on complexTemplateBenchmark (3,316 -> 6,772 ops/s)

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

Author: jasmith-hs

src/main/java/com/hubspot/jinjava/objects/serialization/PyishObjectMapper.java

@@ -57,10 +57,13 @@ private static ObjectMapper getPyishObjectMapper() {
   }
 
   public static String getAsUnquotedPyishString(Object val) {
-    if (val != null) {
-      return WhitespaceUtils.unquoteAndUnescape(getAsPyishString(val, true));
+    if (val == null) {
+      return "";
     }
-    return "";
+    if (val instanceof String || val instanceof Number || val instanceof Boolean) {
+      return val.toString();
+    }
+    return WhitespaceUtils.unquoteAndUnescape(getAsPyishString(val, true));
   }
 
   public static String getAsPyishString(Object val) {