…nit tests (elastic#258974)

Closes elastic#248548

- Migrates flaky `repository_add` client integration test to focused
unit tests spread across the source files that own the functionality.
- Removes the legacy
`__jest__/client_integration/repository_add.test.ts` integration test.
- Unskips the previously flaky settings validation test (now a pure
function test — no timeouts possible).

- The old integration test rendered the full app through a router with
async HTTP mock layers, causing flaky 5000ms timeouts on CI (issue
- The new unit tests render components directly with minimal mocks, use
synchronous `fireEvent`, and test validation as pure function calls.

- [Integration → unit test mapping
(gist)](https://gist.github.com/kapral18/b6ccde9cbf84bc18c35fc356861aba50)

- `node scripts/check_changes.ts` — passed
- `yarn test:jest
x-pack/platform/plugins/private/snapshot_restore/public/application/services/validation/validate_repository.test.ts`
— 24 passed
- `yarn test:jest
x-pack/platform/plugins/private/snapshot_restore/public/application/components/repository_form/step_one.test.tsx`
— 10 passed
- `yarn test:jest
x-pack/platform/plugins/private/snapshot_restore/public/application/components/repository_form/repository_form.test.tsx`
— 14 passed
- `yarn test:jest
x-pack/platform/plugins/private/snapshot_restore/public/application/sections/repository_add/repository_add.test.tsx`
— 1 passed

Assisted with Claude Code using Claude 4.6 Opus

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
(cherry picked from commit 6b62324)