[SPARK-51625][SQL] Command in CTE relations should trigger inline

### What changes were proposed in this pull request?

This PR fixes a small issue in the rule `CTESubstitution`. The intention is to fall back to CTE inline if there are commands in the plan. However, the code simply does `plan.collect` which misses the fact that `UnresolvedWith` does not include the CTE relations in its children.

To fix it, we should do a manually recursion to handle the CTE relations.

### Why are the changes needed?

A potential bug as the code does not work as expected.

### Does this PR introduce _any_ user-facing change?

No, there is no user-facing API that can construct such a CTE plan.

### How was this patch tested?

a new test

### Was this patch authored or co-authored using generative AI tooling?

no

Closes #50425 from cloud-fan/cte.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: cloud-fan

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CTESubstitution.scala

@@ -56,9 +56,15 @@ object CTESubstitution extends Rule[LogicalPlan] {
       return plan
     }
 
-    val commands = plan.collect {
-      case c @ (_: Command | _: ParsedStatement | _: InsertIntoDir) => c
+    def collectCommands(p: LogicalPlan): Seq[LogicalPlan] = p match {
+      case c @ (_: Command | _: ParsedStatement | _: InsertIntoDir) => Seq(c)
+      case u: UnresolvedWith =>
+        collectCommands(u.child) ++ u.cteRelations.flatMap {
+          case (_, relation) => collectCommands(relation)
+        }
+      case p => p.children.flatMap(collectCommands)
     }
+    val commands = collectCommands(plan)
     val forceInline = if (commands.length == 1) {
       if (conf.getConf(SQLConf.LEGACY_INLINE_CTE_IN_COMMANDS)) {
         // The legacy behavior always inlines the CTE relations for queries in commands.

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

@@ -17,6 +17,7 @@
 
 package org.apache.spark.sql
 
+import org.apache.spark.sql.catalyst.analysis.{CurrentNamespace, UnresolvedRelation}
 import org.apache.spark.sql.catalyst.expressions.{Alias, And, GreaterThan, LessThan, Literal, Or, Rand}
 import org.apache.spark.sql.catalyst.optimizer.InlineCTE
 import org.apache.spark.sql.catalyst.plans.logical._
@@ -834,6 +835,17 @@ abstract class CTEInlineSuiteBase
       }
     }
   }
+
+  test("SPARK-51625: command in CTE relations should trigger inline") {
+    val plan = UnresolvedWith(
+      child = UnresolvedRelation(Seq("t")),
+      cteRelations = Seq("t" -> SubqueryAlias("t", ShowTables(CurrentNamespace, pattern = None)))
+    )
+    assert(!spark.sessionState.analyzer.execute(plan).exists {
+      case _: WithCTE => true
+      case _ => false
+    })
+  }
 }
 
 class CTEInlineSuiteAEOff extends CTEInlineSuiteBase with DisableAdaptiveExecutionSuite