Add the failing call's context to StateMachineErrorContext #7

The vscode-liquidjava State Machine view needs more context about a failing call than the parser provides (liquid-java/vscode-liquidjava#117, #155). Support for multiple @StateSet groups is tracked separately in #8.

Error context for the failing call

StateMachineErrorContext holds only calledMethod and actualStates. Keep both and add:

• receiver: the receiver of the failing call;
• expectedStates: the required states from the Expected condition;
• expectedCondition: the Expected condition as text;
• foundCondition: the Found condition for the receiver, as text;
• definiteFoundState: the one Found state, only when Found establishes a single definite state; otherwise null.

Identify the called method by its declaration position, not by label text. Constructors and static factories should also be matched.

Tests

Add fixtures for:

• the DefaultMutableTreeNode example from vscode-liquidjava#117: Found childrenAllowed, required maybeHasChildren;
• a precondition that accepts more than one state;
• a Found condition that does not establish one state.