[#31029] YSQL: Add new initdb test flag to --undefok for upgrade tests

Address PR #31753 CI failures on arm-alma8-clang21-release (120
failed tests, all upgrade-test families). The previous commit added
--TEST_master_min_live_tservers_before_initdb unconditionally to every
master command line in ExternalMiniCluster::StartMaster(). Upgrade tests
boot older yb-master binaries from build/db-upgrade/yugabyte_*/bin which
do not recognize the flag and exit with:

  [m-1] ERROR: unknown command line flag
        'TEST_master_min_live_tservers_before_initdb'
  Failed to start masters.: ...
  /build/db-upgrade/yugabyte_2.25.0.0-b340_linux_release/bin/yb-master

Use AppendCsvFlagValue() to append the flag to the existing --undefok
list, mirroring the pattern in upgrade_test_base.cc and the existing
AppendFlagToAllowedPreviewFlagsCsv() helper in this file. Older masters
silently ignore the flag, which is safe: upgrade-test bootstrap loads a
pre-baked sys-catalog snapshot and bypasses initdb, so the older master
never hits the race the flag was added to fix.

Test plan:
* Compile-checked locally with ./yb_build.sh release --target
  integration-tests.
* CI on arm-alma8-clang21-release (primary signal: the upgrade-test
  family no longer fails with "unknown command line flag").

Author: hkimura

src/yb/integration-tests/external_mini_cluster.cc

@@ -578,6 +578,12 @@ Result<ExternalMasterPtr> ExternalMiniCluster::StartMaster(
     // to start exactly that many tservers immediately after, and (b) the master's RF
     // requirement is min(num_tablet_servers, FLAGS_replication_factor), so any test that
     // survives the bootstrap eventually needs all of them anyway.
+    //
+    // Add the flag to --undefok so that upgrade tests (which boot older yb-master binaries
+    // from build/db-upgrade/) silently ignore it. Older masters never had this race in
+    // upgrade scenarios -- they load a pre-baked sys-catalog snapshot and bypass initdb
+    // altogether -- so being able to skip the flag is safe.
+    AppendCsvFlagValue(flags, "undefok", "TEST_master_min_live_tservers_before_initdb");
     flags.push_back(Format(
         "--TEST_master_min_live_tservers_before_initdb=$0", opts_.num_tablet_servers));
     if (opts_.enable_ysql_auth) {