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_cache.py

@@ -34,13 +34,6 @@ def _make_task_cache(pool: PagePool) -> TaskCacheManager:
 # ---- page_hash ----
 
 
-def test_page_hash_full_page():
-    token_ids = list(range(256))
-    h = page_hash(token_ids, 0, 64)
-    assert isinstance(h, int)
-    assert h >= 0
-
-
 def test_page_hash_different_page_differs():
     token_ids = list(range(256))
     assert page_hash(token_ids, 0, 64) != page_hash(token_ids, 1, 64)
@@ -121,19 +114,13 @@ def test_prefix_cache_ignores_partial_last_page():
 
 def test_prefix_cache_on_evict_clears_mappings():
     prefix = RadixCache(64)
+    assert not prefix.has_page(0)
     prefix.record(0, list(range(64)), 0)
     assert prefix.has_page(0)
     prefix.evict(0)
     assert not prefix.has_page(0)
 
 
-def test_prefix_cache_has_page():
-    prefix = RadixCache(64)
-    assert not prefix.has_page(0)
-    prefix.record(0, list(range(64)), 0)
-    assert prefix.has_page(0)
-
-
 def test_prefix_cache_does_not_reuse_page_without_parent_prefix():
     prefix = RadixCache(2)
     prefix.record(0, [1, 2, 3, 4], 0)