ct/l1/metastore: check abort source when purging

In the rapid topic creation test, it's seen that shutdown hangs with the
topic purger, and I think it is due to we see 100 topics we need to
purge, and with a timeout of 5 seconds each, it's probably pretty easy
to blow past our shutdown timeouts.

I wish our RPC system supported abort_sources.

Author: rockwotj

src/v/cloud_topics/level_one/metastore/topic_purger.cc

@@ -94,7 +94,8 @@ topic_purger::purge_tombstoned_topics(ss::abort_source* as) {
           ss::max_concurrent_for_each(
             ntrs_to_purge,
             max_concurrent_purges,
-            [this, &first_error](const cluster::nt_revision& ntr) {
+            [this, as, &first_error](const cluster::nt_revision& ntr) {
+                as->check();
                 return remove_tombstone_(ntr).then(
                   [&first_error](
                     const topic_purger::remove_tombstone_ret_t& ret) {