Rewrite the integration-test foundations

## Outcome

Integration tests validate user outcomes and provider contracts with reliable, maintainable fixtures.

## Scope

- Inventory and classify existing tests
- Define test layers, environments, fixtures, and ownership
- Align scenarios with refreshed user stories
- Cover provider contracts, metadata failures, API compatibility, and critical user journeys

## Acceptance criteria

- [ ] Scope is decomposed into independently deliverable child issues.
- [ ] Architecture and API decisions are linked from this issue.
- [ ] Test, documentation, observability, rollout, and failure behavior are defined where applicable.
- [ ] Epic exit criteria are verified before closure.

## Planning note

This is a 2026 GovTool revamp work item. Its child features and tasks should be added as the scope is refined.

Start by inventorying and classifying the existing integration tests, then decompose this epic into independently deliverable child issues. Define the test layers, environments, fixtures, ownership, provider contracts, failure cases, API compatibility, and critical user journeys. Done means architecture and API decisions are linked and the test, documentation, observability, rollout, failure behavior, and exit criteria are verified.

Written by the indexing model from the issue text.