Add contract integration fixtures for SDK consumers

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Add deterministic contract fixtures for SDK and app consumers.

Current Behaviour

SDK and mobile tests may need stable examples of contract responses, errors, and state transitions.

Expected Behaviour

The contracts repo should provide fixtures for major vault scenarios.

Suggested Implementation

Create fixtures for deposit, withdraw, lock, balance, errors, and events where applicable.

Files or Areas Likely Affected

• fixtures/
• tests/
• contracts/
• src/
• docs/

Acceptance Criteria

• Contract integration fixtures are added.
• Deposit and withdrawal scenarios are included.
• Locked balance scenarios are included.
• Error scenarios are included.
• Fixtures avoid real user secrets.
• Docs explain fixture usage.

Additional Notes

This supports cross-repo testing quality.