ct: Fix pipeline_stage assertion

Previously the assertion was checking that the new stage id is less than
max stage id. But this is incorrect because not all possible stages are
allocated. The commit changes this so the assertion is only checking if
the next stage id is allocated.

Signed-off-by: Evgeny Lazin <4lazin@gmail.com>

Author: Lazin

src/v/cloud_topics/level_zero/pipeline/pipeline_stage.cc

@@ -34,7 +34,7 @@ pipeline_stage pipeline_stage_container::next_stage(pipeline_stage old) const {
     auto next_ix = old_ix + 1;
     // Check that we have next stage
     vassert(
-      static_cast<size_t>(next_ix) < _stages.size(),
+      static_cast<size_t>(next_ix) < _registered,
       "Pipeline stage {} is not registered",
       next_ix);
     return pipeline_stage(&_stages.at(next_ix));