feat(metamodel): Add incoming and outgoing StateTransitions to State

Register and declare ReferenceSearchingAccessors for StateTransitions
leading into and out of a State:

- incoming_transitions to all *State classes except InitialPseudoState
- outgoing_transitions to all *State classes except FinalState and
  TerminatePseudoState

Refs: DSD-DBS#426

capellambse/metamodel/capellacommon.py

@@ -49,6 +49,9 @@ class State(AbstractStateMode):
         m.ModelElement, "exit", aslist=m.MixedElementList
     )
 
+    incoming_transitions = m.Accessor
+    outgoing_transitions = m.Accessor
+
     functions: m.Accessor
 
 
@@ -161,6 +164,40 @@ def name(self) -> str:  # type: ignore[override]
         ),
     )
 
+for cls in [
+    State,
+    Mode,
+    DeepHistoryPseudoState,
+    FinalState,
+    ForkPseudoState,
+    JoinPseudoState,
+    ShallowHistoryPseudoState,
+    TerminatePseudoState,
+]:
+    m.set_accessor(
+        cls,
+        "incoming_transitions",
+        m.ReferenceSearchingAccessor(
+            StateTransition, "destination", aslist=m.ElementList
+        ),
+    )
+for cls in [
+    State,
+    Mode,
+    DeepHistoryPseudoState,
+    ForkPseudoState,
+    InitialPseudoState,
+    JoinPseudoState,
+    ShallowHistoryPseudoState,
+]:
+    m.set_accessor(
+        cls,
+        "outgoing_transitions",
+        m.ReferenceSearchingAccessor(
+            StateTransition, "source", aslist=m.ElementList
+        ),
+    )
+
 m.set_accessor(
     Region,
     "states",

tests/test_metamodel_common.py

@@ -48,3 +48,26 @@ def test_state_attributes(
     functions = getattr(state, attr)
 
     assert functions == expected_functions
+
+
+def test_state_outgoing_transitions(model: capellambse.MelodyModel) -> None:
+    state = model.by_uuid("1d03b3f0-f65b-451a-b9c7-b5df12b7bf4c")
+    expected_outgoing = [model.by_uuid("8f868115-f5fe-412c-b2a2-4adf92806fc6")]
+
+    assert state.outgoing_transitions == expected_outgoing
+
+
+def test_initial_state_outgoing_transitions(
+    model: capellambse.MelodyModel,
+) -> None:
+    initial_state = model.by_uuid("43932114-8ad4-4074-b2a9-b0d55b8d027b")
+    expected_outgoing = [model.by_uuid("4764929e-92b5-4400-a57c-432477275f48")]
+
+    assert initial_state.outgoing_transitions == expected_outgoing
+
+
+def test_state_incoming_transitions(model: capellambse.MelodyModel) -> None:
+    state = model.by_uuid("1d03b3f0-f65b-451a-b9c7-b5df12b7bf4c")
+    expected_incoming = [model.by_uuid("8770f95c-ac4e-4d37-a690-0920aa22d3f9")]
+
+    assert state.incoming_transitions == expected_incoming