[Spark] Disable implicit casting in Delta streaming sink (#3691)

## Description
#3443 introduced implicit casting
when writing to a Delta table using a streaming query.


We are disabling this change for now as it regresses behavior when a
struct field is missing in the input data. This previously succeeded,
filling the missing fields with `null` but would now fail with:
```
DELTA_UPDATE_SCHEMA_MISMATCH_EXPRESSION] Cannot cast struct<name:string> to struct<name:string,age:bigint>. All nested columns must match.
```

Note: batch INSERT fails in this scenario with:
```
[DELTA_INSERT_COLUMN_ARITY_MISMATCH] Cannot write to '<table>, not enough nested fields in <struct>; target table has 3 column(s) but the inserted data has 2 column(s)
```
but since streaming write allowed this, we have to preserve that
behavior.

## How was this patch tested?
Tests added as part of #3443, e.p.
with flag disabled.

## Does this PR introduce _any_ user-facing changes?
Disabled behavior change that was to be introduced with
#3443.

spark/src/main/scala/org/apache/spark/sql/delta/sources/DeltaSQLConf.scala

@@ -1561,7 +1561,9 @@ trait DeltaSQLConfBase {
           |The casting behavior is governed by 'spark.sql.storeAssignmentPolicy'.
           |""".stripMargin)
       .booleanConf
-      .createWithDefault(true)
+      // This feature doesn't properly support structs with missing fields and is disabled until a
+      // fix is implemented.
+      .createWithDefault(false)
 
   val DELTA_CDF_UNSAFE_BATCH_READ_ON_INCOMPATIBLE_SCHEMA_CHANGES =
     buildConf("changeDataFeed.unsafeBatchReadOnIncompatibleSchemaChanges.enabled")

spark/src/test/scala/org/apache/spark/sql/delta/DeltaInsertIntoImplicitCastSuite.scala

@@ -17,8 +17,8 @@
 package org.apache.spark.sql.delta
 
 import org.apache.spark.sql.delta.sources.DeltaSQLConf
-
 import org.apache.spark.sql.SaveMode
+import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types._
 
 /**
@@ -32,6 +32,12 @@ import org.apache.spark.sql.types._
  */
 class DeltaInsertIntoImplicitCastSuite extends DeltaInsertIntoTest {
 
+  override def beforeAll(): Unit = {
+    super.beforeAll()
+    spark.conf.set(DeltaSQLConf.DELTA_STREAMING_SINK_ALLOW_IMPLICIT_CASTS.key, "true")
+    spark.conf.set(SQLConf.ANSI_ENABLED.key, "true")
+  }
+
   for (schemaEvolution <- BOOLEAN_DOMAIN) {
     testInserts("insert with implicit up and down cast on top-level fields, " +
       s"schemaEvolution=$schemaEvolution")(