Support for async node functions (#8830)

* Support for async execution functions

This commit adds support for node execution functions defined as async. When
a node's execution function is defined as async, we can continue
executing other nodes while it is processing.

Standard uses of `await` should "just work", but people will still have
to be careful if they spawn actual threads. Because torch doesn't really
have async/await versions of functions, this won't particularly help
with most locally-executing nodes, but it does work for e.g. web
requests to other machines.

In addition to the execute function, the `VALIDATE_INPUTS` and
`check_lazy_status` functions can also be defined as async, though we'll
only resolve one node at a time right now for those.

* Add the execution model tests to CI

* Add a missing file

It looks like this got caught by .gitignore? There's probably a better
place to put it, but I'm not sure what that is.

* Add the websocket library for automated tests

* Add additional tests for async error cases

Also fixes one bug that was found when an async function throws an error
after being scheduled on a task.

* Add a feature flags message to reduce bandwidth

We now only send 1 preview message of the latest type the client can
support.

We'll add a console warning when the client fails to send a feature
flags message at some point in the future.

* Add async tests to CI

* Don't actually add new tests in this PR

Will do it in a separate PR

* Resolve unit test in GPU-less runner

* Just remove the tests that GHA can't handle

* Change line endings to UNIX-style

* Avoid loading model_management.py so early

Because model_management.py has a top-level `logging.info`, we have to
be careful not to import that file before we call `setup_logging`. If we
do, we end up having the default logging handler registered in addition
to our custom one.

tests-unit/websocket_feature_flags_test.py

@@ -0,0 +1,77 @@
+"""Simplified tests for WebSocket feature flags functionality."""
+from comfy_api import feature_flags
+
+
+class TestWebSocketFeatureFlags:
+    """Test suite for WebSocket feature flags integration."""
+
+    def test_server_feature_flags_response(self):
+        """Test server feature flags are properly formatted."""
+        features = feature_flags.get_server_features()
+
+        # Check expected server features
+        assert "supports_preview_metadata" in features
+        assert features["supports_preview_metadata"] is True
+        assert "max_upload_size" in features
+        assert isinstance(features["max_upload_size"], (int, float))
+
+    def test_progress_py_checks_feature_flags(self):
+        """Test that progress.py checks feature flags before sending metadata."""
+        # This simulates the check in progress.py
+        client_id = "test_client"
+        sockets_metadata = {"test_client": {"feature_flags": {}}}
+
+        # The actual check would be in progress.py
+        supports_metadata = feature_flags.supports_feature(
+            sockets_metadata, client_id, "supports_preview_metadata"
+        )
+
+        assert supports_metadata is False
+
+    def test_multiple_clients_different_features(self):
+        """Test handling multiple clients with different feature support."""
+        sockets_metadata = {
+            "modern_client": {
+                "feature_flags": {"supports_preview_metadata": True}
+            },
+            "legacy_client": {
+                "feature_flags": {}
+            }
+        }
+
+        # Check modern client
+        assert feature_flags.supports_feature(
+            sockets_metadata, "modern_client", "supports_preview_metadata"
+        ) is True
+
+        # Check legacy client
+        assert feature_flags.supports_feature(
+            sockets_metadata, "legacy_client", "supports_preview_metadata"
+        ) is False
+
+    def test_feature_negotiation_message_format(self):
+        """Test the format of feature negotiation messages."""
+        # Client message format
+        client_message = {
+            "type": "feature_flags",
+            "data": {
+                "supports_preview_metadata": True,
+                "api_version": "1.0.0"
+            }
+        }
+
+        # Verify structure
+        assert client_message["type"] == "feature_flags"
+        assert "supports_preview_metadata" in client_message["data"]
+
+        # Server response format (what would be sent)
+        server_features = feature_flags.get_server_features()
+        server_message = {
+            "type": "feature_flags",
+            "data": server_features
+        }
+
+        # Verify structure
+        assert server_message["type"] == "feature_flags"
+        assert "supports_preview_metadata" in server_message["data"]
+        assert server_message["data"]["supports_preview_metadata"] is True