test(validation): add contract, canary, and round-trip tests for adapters

Add 119 comprehensive tests across 5 test files to validate test
infrastructure and prevent adapter bugs:

Contract Tests (81 tests):
- benchmark-graph-expander.unit.test.ts (28 tests)
- test-graph-expander.unit.test.ts (26 tests)
- instrumented-expander.unit.test.ts (27 tests)

Canary Integration Tests (18 tests):
- canary-tests.integration.test.ts

Round-Trip Tests (20 tests):
- adapter-roundtrip.unit.test.ts

These tests specifically prevent the BenchmarkGraphExpander reverse edge
bug that caused universal false negatives in coverage metrics.

Author: Mearman