test: achieve 100% branch coverage in ParsingContext

[Copilot]

Branch coverage was at 99.87% due to the else path of if ('@id' in contextKeyEntry) in ParsingContext.getContext never being exercised — all existing property-scoped context tests included an @id on the scoped entry.

Change

Added a test case where a property-scoped context entry has @context but no @id, covering the previously untested branch:

// ParsingContext.ts:250 — previously only the true branch was hit
if ('@id' in contextKeyEntry) {
  contextRaw[key]['@id'] = contextKeyEntry['@id'];
}

The new test sets up a root context with a property a that carries a @context sub-context but no @id, then asserts that getContext resolves with the scoped vocab and an empty entry for a.