test: create api fixture validator

[davidcreated]

Summary

Implements an API Fixture Validator that checks the JSON fixtures under e2e/fixtures/ against Zod schemas mirroring the current API/domain shapes (the canonical types in frontend/src/types). This catches fixture drift — fields added, removed, renamed, or retyped in the API but not in the fixtures — before stale fixtures silently weaken the tests.

Closes #561

What's included

• Validator core (backend/src/testing/fixtureValidator/)
  • schemas.ts — Zod schemas mirroring Asset, HealthScore, HealthFactors, Bridge, and the fixture-file envelopes.
  • registry.ts — maps each fixture file to the schema describing its API shape.
  • validator.ts — reads, parses, and validates fixtures; emits readable, path-pointed diffs (e.g. USDC.factors.liquidityDepth).
  • format.ts — CI-friendly text report.
• Severity model
  • error — missing/mistyped required field, invalid enum, unreadable/invalid JSON → fails CI.
  • warning — a property no longer part of the current API shape → fails only under --strict.
• CLI — npm --workspace=backend run fixtures:validate (supports --json and --strict), exits non-zero on drift.
• CI — runs in the node-ci job right after lint.
• Unit tests — backend/tests/testing/fixtureValidator.test.ts (validates every registered fixture + covers missing field, wrong type, bad enum, unexpected property, and nested record paths).
• Docs — backend/docs/FIXTURE_VALIDATION.md covering how to run it, the update workflow when the API changes, and how to add a new fixture.

Drift fixed in this PR

While building the validator it flagged real drift: e2e/fixtures/asset-health.json was missing the required HealthScore.lastUpdated field. Fixed so the committed fixtures match the current shape.

Verification

• npm --workspace=backend run lint ✅
• npm --workspace=backend run build ✅
• npm --workspace=backend run fixtures:validate → PASS ✅
• npm --workspace=backend run test -- tests/testing → 9/9 passing ✅
• Confirmed the validator fails with readable diffs + exit code 1 when a fixture is intentionally broken.

How to extend

Add a fixture file, add a schema to schemas.ts, and register it in registry.ts — it's then covered by both the CLI check and the test suite.