test(validation): negative load-error checks; keep run logs out of the repo

load_errors_check.py loads deliberately broken .tum fixtures in batch on the
build under test (like lsp_check.py) and asserts each fails with its specific
located message and without a raw traceback. Wired into run.sh just before the
main suite, so it runs for every channel.

The run validation items now point their sub-instance log at the gitignored
report dir, so a GUI run no longer litters the tree with sub_*.log files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/validation/load_errors/group_no_steps.tum

@@ -0,0 +1,6 @@
+main:
+  name: root
+  steps:
+    # A container item (group) without its mandatory 'steps:' list.
+    - group:
+        name: g