chore(audit): audit json expressions across Spark 3.4.3, 3.5.8, 4.0.1, 4.1.1 (apache#4470)

Author: andygrove

docs/source/contributor-guide/spark_expressions_support.md

@@ -511,6 +511,11 @@
 
 - [ ] from_json
 - [x] get_json_object
+  - Spark 3.4.3 (audited 2026-05-27): identical to 3.5.8.
+  - Spark 3.5.8 (audited 2026-05-27): baseline. `BinaryExpression with ExpectsInputTypes with CodegenFallback`; `inputTypes = Seq(StringType, StringType) -> StringType`. Eval is inline and uses Jackson with `RawStyle` output. Foldable paths are parsed once. Returns NULL for invalid JSON, missing paths, or `JsonProcessingException`.
+  - Spark 4.0.1 (audited 2026-05-27): the eval is extracted into a `GetJsonObjectEvaluator` helper (no behaviour change). The trait set now mixes in `DefaultStringProducingExpression`, and `inputTypes` is widened to `StringTypeWithCollation(supportsTrimCollation = true)` for both arguments.
+  - Spark 4.1.1 (audited 2026-05-27): identical to 4.0.1.
+  - Known incompatibility: Spark accepts single-quoted JSON and unescaped control characters; Comet's native parser (built on `serde_json`) rejects both, so those inputs require `spark.comet.expression.GetJsonObject.allowIncompatible=true` and may still produce different results. Non-default Spark 4.0 string collations are not propagated (https://github.com/apache/datafusion-comet/issues/2190).
 - [ ] json_array_length
 - [ ] json_object_keys
 - [ ] json_tuple

spark/src/main/scala/org/apache/comet/serde/strings.scala

@@ -443,15 +443,14 @@ object CometStringSplit extends CometExpressionSerde[StringSplit] {
 
 object CometGetJsonObject extends CometExpressionSerde[GetJsonObject] {
 
-  override def getIncompatibleReasons(): Seq[String] = Seq(
+  private val incompatReason =
     "Spark allows single-quoted JSON and unescaped control characters which Comet does not" +
-      " support")
+      " support"
+
+  override def getIncompatibleReasons(): Seq[String] = Seq(incompatReason)
 
   override def getSupportLevel(expr: GetJsonObject): SupportLevel =
-    Incompatible(
-      Some(
-        "Spark allows single-quoted JSON and unescaped control characters " +
-          "which Comet does not support"))
+    Incompatible(Some(incompatReason))
 
   override def convert(
       expr: GetJsonObject,