feat: async engine parity, multi-target transitions, and SCXML send fixes

- Rewrite AsyncEngine to mirror SyncEngine's 3-phase macrostep
  architecture with full StateChart support
- Extract pure computation helpers from BaseEngine so both engines
  share the same logic
- Add on_error callback support to async dispatch methods
- Support multi-target initial transitions for SCXML parallel regions
  in engine, IO layer, parser, and factory
- Fix SCXML send error handling: error.communication for undispatchable
  session targets, error.execution for invalid targets/types, and
  namelist variable validation
- Remove resolved xfail markers (test364, test496, test521, test553)

Author: fgmacedo

AGENTS.md

@@ -51,8 +51,15 @@ uv run pytest tests/test_signature.py::TestSignatureAdapter::test_wrap_fn_single
 uv run pytest -m "not slow"
 ```
 
-Tests include doctests from both source modules (`--doctest-modules`) and markdown docs
-(`--doctest-glob=*.md`). Coverage is enabled by default.
+When trying to run all tests, prefer to use xdist (`-n`) as some SCXML tests uses timeout of 30s to verify fallback mechanism.
+Don't specify the directory `tests/`, because this will exclude doctests from both source modules (`--doctest-modules`) and markdown docs
+(`--doctest-glob=*.md`) (enabled by default):
+
+```bash
+uv run pytest -n auto
+```
+
+Coverage is enabled by default.
 
 ## Linting and formatting
 

statemachine/callbacks.py

@@ -298,20 +298,39 @@ def add(self, key: str, spec: CallbackSpec, builder: Callable[[], Callable]):
 
         insort(self.items, wrapper)
 
-    async def async_call(self, *args, **kwargs):
-        return await asyncio.gather(
-            *(
-                callback(*args, **kwargs)
-                for callback in self
-                if callback.condition(*args, **kwargs)
+    async def async_call(
+        self, *args, on_error: "Callable[[Exception], None] | None" = None, **kwargs
+    ):
+        if on_error is None:
+            return await asyncio.gather(
+                *(
+                    callback(*args, **kwargs)
+                    for callback in self
+                    if callback.condition(*args, **kwargs)
+                )
             )
-        )
 
-    async def async_all(self, *args, **kwargs):
-        coros = [condition(*args, **kwargs) for condition in self]
-        for coro in asyncio.as_completed(coros):
-            if not await coro:
-                return False
+        results = []
+        for callback in self:
+            if callback.condition(*args, **kwargs):
+                try:
+                    results.append(await callback(*args, **kwargs))
+                except Exception as e:
+                    on_error(e)
+        return results
+
+    async def async_all(
+        self, *args, on_error: "Callable[[Exception], None] | None" = None, **kwargs
+    ):
+        for callback in self:
+            try:
+                if not await callback(*args, **kwargs):
+                    return False
+            except Exception as e:
+                if on_error is not None:
+                    on_error(e)
+                    return False
+                raise
         return True
 
     def call(self, *args, on_error: "Callable[[Exception], None] | None" = None, **kwargs):
@@ -388,8 +407,16 @@ def call(
             return []
         return self._registry[key].call(*args, on_error=on_error, **kwargs)
 
-    def async_call(self, key: str, *args, **kwargs):
-        return self._registry[key].async_call(*args, **kwargs)
+    async def async_call(
+        self,
+        key: str,
+        *args,
+        on_error: "Callable[[Exception], None] | None" = None,
+        **kwargs,
+    ):
+        if key not in self._registry:
+            return []
+        return await self._registry[key].async_call(*args, on_error=on_error, **kwargs)
 
     def all(
         self,
@@ -402,8 +429,16 @@ def all(
             return True
         return self._registry[key].all(*args, on_error=on_error, **kwargs)
 
-    def async_all(self, key: str, *args, **kwargs):
-        return self._registry[key].async_all(*args, **kwargs)
+    async def async_all(
+        self,
+        key: str,
+        *args,
+        on_error: "Callable[[Exception], None] | None" = None,
+        **kwargs,
+    ):
+        if key not in self._registry:
+            return True
+        return await self._registry[key].async_all(*args, on_error=on_error, **kwargs)
 
     def str(self, key: str) -> str:
         if key not in self._registry: