[#29239] YSQL: Guardrail to prevent autonomous txn in transaction block

Summary:
Some DDLs can run as autonomous transactions (i.e., not part of a
transaction block) even if transactional DDL mode
(ysql_yb_ddl_transaction_block_enabled) is on. Currently, the only
example is `CREATE INDEX CONCURRENTLY`.

Autonomous DDL transactions use the kDDL session on the local tserver proxy.
Currently, it is not possible to run an autonomous DDL transaction while a
regular transaction block is active (which uses the kPlain session on the local
tserver proxy). Some of our code relies on this fact. This revision adds
a check to ensure that this assumption is not violated.
Jira: DB-19011

Test Plan: Jenkins

Reviewers: stiwary

Reviewed By: stiwary

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D48027

Author: pkj415

src/postgres/src/backend/utils/misc/pg_yb_utils.c

@@ -2647,6 +2647,12 @@ YbTrackAlteredTableId(Oid relid)
 void
 YBIncrementDdlNestingLevel(YbDdlMode mode)
 {
+	if (YBIsDdlTransactionBlockEnabled() && IsTransactionBlock())
+	{
+		elog(ERROR,
+				"YBIncrementDdlNestingLevel: autonomous DDL not exepcted inside a transaction block");
+	}
+
 	if (ddl_transaction_state.nesting_level == 0)
 	{
 		/*