M2: contract-test infrastructure consolidation — shared scaffold, single Support classes (R2-15) #34

From the second verified review of PR #26 (finding R2-15, CONFIRMED). Pure test-infrastructure hygiene; testing-style.md is the spec.

Finding. The M2 contract tests violate the repo's own testing-style rules: the Endpoint/CreateClient/JsonResponse scaffold is copy-pasted verbatim across three contract-test classes; identical wire literals repeat across test methods and files (the {"_tag":"UnknownError",...} and {"_tag":"SessionNotFoundError",...} bodies, the {"data":...} envelope wrapper 6+ times); FixtureLoader exists in Extensions.Tests as a namespace-only byte-copy of the Sdk.Tests one; RecordingHttpHandler exists there as a divergent same-named twin — drift already visible between the copies.

Shape of the fix: one shared contract-test scenario/support home per testing-style.md §1 (central scaffold, domain-aware helpers, wire bodies in fixtures or centralized constants); a single FixtureLoader/RecordingHttpHandler pair reachable from both test projects (shared source or a test-support project — pick whichever the csproj topology makes cleanest without weakening the IVT posture). Also add the missing object-envelope {"data":null} contract test the review verified as working-but-unobserved.

Trigger: the next breadth batch multiplies contract tests; consolidate before the third copy of the scaffold appears.

Milestone: M2 (lands as its own small PR right after PR #26 merges, before the next breadth batch opens).