Map CLEAR_PROJECTION to DROP_PROJECTION in EXPLAIN AST output

ClickHouse EXPLAIN AST normalizes CLEAR PROJECTION to DROP_PROJECTION,
similar to how CLEAR_COLUMN→DROP_COLUMN and CLEAR_INDEX→DROP_INDEX
are already mapped.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

internal/explain/statements.go

@@ -1684,6 +1684,10 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 	if cmdType == ast.AlterClearIndex {
 		cmdType = ast.AlterDropIndex
 	}
+	// CLEAR_PROJECTION is shown as DROP_PROJECTION in EXPLAIN AST
+	if cmdType == ast.AlterClearProjection {
+		cmdType = ast.AlterDropProjection
+	}
 	// DELETE_WHERE is shown as DELETE in EXPLAIN AST
 	if cmdType == ast.AlterDeleteWhere {
 		cmdType = "DELETE"

parser/testdata/01701_clear_projection_and_part_remove/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt8": true
-  }
-}
+{}

parser/testdata/01710_projection_fetch_long/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt19": true
-  }
-}
+{}