test(backend): establish contract-compatible API fixtures and consumer checks

[arisu6804]

Problem

Client behavior is vulnerable to unnoticed changes in error shapes, precision fields, pagination metadata, and transaction status values.

Objective

Deliver a production-quality improvement to API schemas, fixtures, and service consumers that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Create versioned success and error fixtures for core vault operations.
• Validate required fields, enum values, numeric precision, and pagination contracts in CI.
• Exercise at least one realistic consumer against the generated contract.

Acceptance criteria

• Breaking response changes fail CI with an actionable diff.
• Fixtures cover success, validation, authorization, provider failure, and pending states.
• The checks run deterministically without external chain access.

Required validation

• Schema validation, consumer contract tests, and CI execution using isolated deterministic fixtures.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused on this issue and explain design tradeoffs.
• Include test evidence, compatibility impact, and migration or rollout notes where relevant.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.