[SPARK-50904][SQL] Fix collation expression walker query execution

### What changes were proposed in this pull request?
Changing when we collect results in `CollationExpressionWalkerSuite` on borders of changing session default collation.

### Why are the changes needed?
Because we are analyzing the query and then setting a different default collation, this leads to problems for runtime replaceable expressions which will only be resolved when we call `collect` and by then the session default collation is changed. This is blocking the change in #49576.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
This is a test-only change.

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #49586 from stefankandic/fixExpressionWalker.

Authored-by: Stefan Kandic <[email protected]>
Signed-off-by: Max Gekk <[email protected]>

sql/core/src/test/scala/org/apache/spark/sql/CollationExpressionWalkerSuite.scala

@@ -740,10 +740,10 @@ class CollationExpressionWalkerSuite extends SparkFunSuite with SharedSparkSessi
     for (funInfo <- funInfos.filter(f => !toSkip.contains(f.getName))) {
       for (query <- "> .*;".r.findAllIn(funInfo.getExamples).map(s => s.substring(2))) {
         try {
-          val resultUTF8 = sql(query)
+          val resultUTF8 = sql(query).collect()
           withSQLConf(SqlApiConf.DEFAULT_COLLATION -> "UTF8_LCASE") {
-            val resultUTF8Lcase = sql(query)
-            assert(resultUTF8.collect() === resultUTF8Lcase.collect())
+            val resultUTF8Lcase = sql(query).collect()
+            assert(resultUTF8 === resultUTF8Lcase)
           }
         } catch {
           case e: SparkRuntimeException => assert(e.getCondition == "USER_RAISED_EXCEPTION")