test: add usedBy matcher / allow scope and origin to be undefined

[nikku]

Proposed Changes

Preparing for the introduction of variables without a scope, that are just used but not produced anywhere (#74) this adjusts our test helper:

• To validate usedBy (containing element IDs)
• To verify non-existing scope (variables that are only used, but produced no-where)

Examples

Verify that a variable is used in two places:

expect(variables).to.variableInclude({
  name: 'isCaseApprovded',
  usedBy: [ 'SequenceFlow_1', 'SequenceFlow_2' ]
});

Verify that scope is not defined via

expect(variables).to.variableInclude({
  name: 'usedButNotDefinedVariable',
  scope: undefined
});

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[nikku]

@Buckwich ready for your review - preparation for what follows in #74.

[Copilot]

Pull request overview

Updates the test assertion helpers to support upcoming “input requirement” variables that may not have scope/origin, and to allow validating usedBy references.

Changes:

• Switch matching logic from isDefined(...) to has(expectedVariable, ...) so tests can explicitly assert scope: undefined / origin: undefined.
• Add support for asserting usedBy membership and length.
• Keep existing variable matching helpers (variableEqual / variableInclude) behavior with the extended matching rules.

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[Copilot]

The new has(expectedVariable, 'scope') behavior in assertVariableMatches allows asserting scope: undefined, but findVariable still uses isDefined(scope) when selecting a candidate. That means a pattern with scope: undefined won’t filter by scope at all and may match the wrong variable when multiple variables share the same name in different scopes, causing false failures. Align findVariable with the new semantics (distinguish “key not provided” vs “provided as undefined”) so it can specifically match variables with no scope when scope is present but undefined.