Build an integration test harness against mocked contract fixtures

[David-patrick-chuks]

Summary

Add a more realistic integration-style testing layer using reusable contract fixtures rather than only narrow unit mocks.

Why this matters

The current unit suite verifies important behavior, but a fixture-backed harness will better protect the SDK from regressions as features and models expand.

Scope

• Create a test harness for fixture-driven request and response scenarios.
• Organize reusable fixture data for multiple modules.
• Ensure tests exercise the SDK through public client entrypoints where possible.
• Keep contributor ergonomics strong.

Acceptance Criteria

• A reusable integration-style harness exists.
• At least two client modules are covered by fixture-driven tests.
• Fixtures are organized clearly for contributor reuse.
• Test documentation explains how to add new fixture cases.

Dependencies

• Depends on Align core domain models with Lily backend response envelopes #1