feat(loaders): add control_after_generate random model selection to checkpoint/unet loaders

The Checkpoint/Unet Loader (LoraManager) nodes now support ComfyUI's
built-in control_after_generate mechanism on the ckpt_name/unet_name combos,
letting users pick a random model on every queue with the selected model
written back into the widget (visible, and lockable via the 'fixed' mode).

A base_model input narrows the random pool: a front-end extension fetches
the name/base_model mapping from the new /api/lm/checkpoints/loader-pool
endpoint and filters the combo options, wired through the node callback,
the refreshComboInNodes extension hook, and a graph.onConfigure hook
installed from onAdded (onNodeCreated fires before the node is attached to
a graph, so the graph reference is unavailable there).

tests/routes/test_random_loader_pool.py

@@ -0,0 +1,89 @@
+"""Tests for the loader-pool endpoint backing the Random Checkpoint/Unet
+Loader nodes' front-end base_model filtering.
+"""
+
+import json
+
+import pytest
+
+from py.routes.checkpoint_routes import CheckpointRoutes
+from py.services.service_registry import ServiceRegistry
+
+
+class _FakeCache:
+    def __init__(self, raw_data):
+        self.raw_data = raw_data
+
+
+class _FakeScanner:
+    def __init__(self, raw_data, model_roots):
+        self._raw_data = raw_data
+        self._model_roots = model_roots
+
+    async def get_cached_data(self, force_refresh=False):
+        return _FakeCache(self._raw_data)
+
+    def get_model_roots(self):
+        return self._model_roots
+
+
+class DummyRequest:
+    def __init__(self, query=None):
+        self.query = query or {}
+
+
+@pytest.fixture
+def routes(tmp_path, monkeypatch):
+    existing = tmp_path / "flux.safetensors"
+    existing.write_bytes(b"x")
+    missing = tmp_path / "missing.safetensors"  # referenced but never created
+
+    raw_data = [
+        {"sub_type": "checkpoint", "file_path": str(existing), "base_model": "Flux.1 D"},
+        {"sub_type": "checkpoint", "file_path": str(missing), "base_model": "SDXL 1.0"},
+        {
+            "sub_type": "diffusion_model",
+            "file_path": str(existing),
+            "base_model": "Flux.1 D",
+        },
+    ]
+
+    async def _fake_scanner():
+        return _FakeScanner(raw_data, [str(tmp_path)])
+
+    monkeypatch.setattr(ServiceRegistry, "get_checkpoint_scanner", _fake_scanner)
+    return CheckpointRoutes()
+
+
+async def test_loader_pool_checkpoint_subtype(routes):
+    response = await routes.get_loader_pool(DummyRequest(query={"sub_type": "checkpoint"}))
+    assert response.status == 200
+    payload = json.loads(response.text)
+    assert payload == {
+        "items": [{"name": "flux.safetensors", "base_model": "Flux.1 D"}]
+    }
+
+
+async def test_loader_pool_diffusion_model_subtype(routes):
+    response = await routes.get_loader_pool(
+        DummyRequest(query={"sub_type": "diffusion_model"})
+    )
+    assert response.status == 200
+    payload = json.loads(response.text)
+    assert payload == {
+        "items": [{"name": "flux.safetensors", "base_model": "Flux.1 D"}]
+    }
+
+
+async def test_loader_pool_default_subtype_is_checkpoint(routes):
+    response = await routes.get_loader_pool(DummyRequest())
+    assert response.status == 200
+    payload = json.loads(response.text)
+    assert payload == {
+        "items": [{"name": "flux.safetensors", "base_model": "Flux.1 D"}]
+    }
+
+
+async def test_loader_pool_invalid_subtype(routes):
+    response = await routes.get_loader_pool(DummyRequest(query={"sub_type": "lora"}))
+    assert response.status == 400