Check readonly mode in spock_alter_subscription_synchronize()

spock_alter_subscription_synchronize() iterates remote tables and calls
truncate_table() when truncate=true, but unlike spock_resync_table() it
was missing the check_readonly_for_resync() guard introduced in
SPOC-440. A readonly subscriber would have its tables truncated with no
way to refill them.

Add the guard and fix the missing forward declaration for
check_readonly_for_resync() (static function defined after its first
call site).

Co-Authored-By: Asif Rehman <asifr@pgedge.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/spock_functions.c

@@ -196,6 +196,7 @@ extern void gen_slot_name(Name slot_name, char *dbname,
 						  const char *provider_name,
 						  const char *subscriber_name);
 static XLogRecPtr skip_wal_records_decoding(bool enable);
+static void check_readonly_for_resync(const char *nspname, const char *relname);
 
 
 bool		in_spock_replicate_ddl_command = false;
@@ -957,6 +958,8 @@ spock_alter_subscription_synchronize(PG_FUNCTION_ARGS)
 		{
 			SpockSyncStatus newsync;
 
+			check_readonly_for_resync(remoterel->nspname, remoterel->relname);
+
 			memset(&newsync, 0, sizeof(SpockSyncStatus));
 			newsync.kind = SYNC_KIND_DATA;
 			newsync.subid = sub->id;