Add test coverage for resolve_expert_dims

Covers the default-config 0-sentinel inheritance path (the exact bug
fixed by 969c5c6, previously untested since every router test in
test_router.py passes expert_hidden_dim/expert_n_blocks explicitly),
plus missing-key inheritance, full override, and partial override.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_config.py

@@ -115,3 +115,33 @@ def test_warn_if_checkpoint_config_mismatch_no_warning_when_hashes_match(
 
     gconfig.warn_if_checkpoint_config_mismatch(ckpt_path)
     assert capsys.readouterr().err == ""
+
+
+def test_resolve_expert_dims_default_config_inherits_hidden_dim_and_n_blocks():
+    # The unset sentinel (expert_hidden_dim/n_blocks == 0 in DEFAULT_CONFIG)
+    # is exactly the bug fixed by resolve_expert_dims: it must not silently
+    # fall back to some other hardcoded default, only to the monolith's own
+    # hidden_dim/n_blocks, so --hidden-dim/--n-blocks reach the experts too.
+    router_cfg = dict(gconfig.DEFAULT_CONFIG["model"]["router"])
+    assert router_cfg["expert_hidden_dim"] == 0
+    assert router_cfg["expert_n_blocks"] == 0
+
+    hidden_dim, n_blocks = gconfig.resolve_expert_dims(router_cfg, 512, 6)
+    assert (hidden_dim, n_blocks) == (512, 6)
+
+
+def test_resolve_expert_dims_missing_keys_also_inherit():
+    hidden_dim, n_blocks = gconfig.resolve_expert_dims({}, 512, 6)
+    assert (hidden_dim, n_blocks) == (512, 6)
+
+
+def test_resolve_expert_dims_explicit_override_wins():
+    router_cfg = {"expert_hidden_dim": 128, "expert_n_blocks": 3}
+    hidden_dim, n_blocks = gconfig.resolve_expert_dims(router_cfg, 512, 6)
+    assert (hidden_dim, n_blocks) == (128, 3)
+
+
+def test_resolve_expert_dims_partial_override_mixes_explicit_and_inherited():
+    router_cfg = {"expert_hidden_dim": 128, "expert_n_blocks": 0}
+    hidden_dim, n_blocks = gconfig.resolve_expert_dims(router_cfg, 512, 6)
+    assert (hidden_dim, n_blocks) == (128, 6)  # n_blocks inherited, hidden_dim not