[SPARK-55865][SQL] Rename _LEGACY_ERROR_TEMP_1266 to CANNOT_TRUNCATE_EXTERNAL_TABLE

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

Rename the legacy error condition `_LEGACY_ERROR_TEMP_1266` to a proper error class `CANNOT_TRUNCATE_EXTERNAL_TABLE` with SQL state `0A000` (feature not supported). Also renames the message parameter from `tableIdentWithDB` to `tableName` for consistency.

### Why are the changes needed?

`_LEGACY_ERROR_TEMP_1266` is a legacy placeholder error name. It should be replaced with a descriptive error class name and assigned a proper SQL state to improve error reporting and comply with the ANSI SQL standard.

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

Yes. The error condition name changes from `_LEGACY_ERROR_TEMP_1266` to `CANNOT_TRUNCATE_EXTERNAL_TABLE` and now includes SQL state `0A000`. The error message itself remains unchanged.

### How was this patch tested?

Existing tests (including `TruncateTableSuite`).

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

Generated-by: Claude Code 2.1.70

Closes #54661 from mikhailnik-db/cannot-truncate-external-table.

Authored-by: Mikhail Nikoliukin <mikhail.nikoliukin@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: mikhailnik-db

common/utils/src/main/resources/error/error-conditions.json

@@ -547,6 +547,12 @@
     ],
     "sqlState" : "58030"
   },
+  "CANNOT_TRUNCATE_EXTERNAL_TABLE" : {
+    "message" : [
+      "Operation not allowed: TRUNCATE TABLE on external tables: <tableName>."
+    ],
+    "sqlState" : "0A000"
+  },
   "CANNOT_UPDATE_FIELD" : {
     "message" : [
       "Cannot update <table> field <fieldName> type:"
@@ -9154,11 +9160,6 @@
       "LOAD DATA target table <tableIdentWithDB> is not partitioned, but a partition spec was provided."
     ]
   },
-  "_LEGACY_ERROR_TEMP_1266" : {
-    "message" : [
-      "Operation not allowed: TRUNCATE TABLE on external tables: <tableIdentWithDB>."
-    ]
-  },
   "_LEGACY_ERROR_TEMP_1267" : {
     "message" : [
       "Operation not allowed: TRUNCATE TABLE ... PARTITION is not supported for tables that are not partitioned: <tableIdentWithDB>."

sql/catalyst/src/main/scala/org/apache/spark/sql/errors/QueryCompilationErrors.scala

@@ -3175,8 +3175,8 @@ private[sql] object QueryCompilationErrors extends QueryErrorsBase with Compilat
 
   def truncateTableOnExternalTablesError(tableIdentWithDB: String): Throwable = {
     new AnalysisException(
-      errorClass = "_LEGACY_ERROR_TEMP_1266",
-      messageParameters = Map("tableIdentWithDB" -> tableIdentWithDB))
+      errorClass = "CANNOT_TRUNCATE_EXTERNAL_TABLE",
+      messageParameters = Map("tableName" -> tableIdentWithDB))
   }
 
   def truncateTablePartitionNotSupportedForNotPartitionedTablesError(

sql/core/src/test/scala/org/apache/spark/sql/execution/command/v1/TruncateTableSuite.scala

@@ -201,8 +201,8 @@ class TruncateTableSuite extends TruncateTableSuiteBase with CommandSuiteBase {
           exception = intercept[AnalysisException] {
             sql(s"TRUNCATE TABLE $t")
           },
-          condition = "_LEGACY_ERROR_TEMP_1266",
-          parameters = Map("tableIdentWithDB" -> "`spark_catalog`.`ns`.`tbl`")
+          condition = "CANNOT_TRUNCATE_EXTERNAL_TABLE",
+          parameters = Map("tableName" -> "`spark_catalog`.`ns`.`tbl`")
         )
       }
     }