test: deduplicate suites and prune low-value cases

- extract shared helpers for dataset writers, scheduler construction, thread interleaving, hf roundtrips, and moe configs
- remove about 20 cases whose only assertions were format checks, restated declarations, fake-taxonomy duplicates, or test-local scaffolding
- strengthen weak cases into exact reference comparisons, positional mask checks, and deterministic outcomes
- replace two schedule factory smoke tests with cosine/sgdr formula assertions
- delete root-level CLI tests whose merge-priority facts are covered by tests/config/test_cli.py
- suite shrinks from 857 to 826 items; ruff format, import order, and pytest all green

tests/inference/test_engine.py

@@ -20,12 +20,6 @@ def _make_engine_mocks(decode=None):
     return mock_model, mock_tokenizer
 
 
-def test_result_append_single():
-    r = GenerateResult(count=1)
-    r.append("hello", 0)
-    assert r.results[0] == "hello"
-
-
 def test_result_append_multiple_tasks():
     r = GenerateResult(count=3)
     r.append("a", 0)