[Test] Add HTTP-layer test coverage for Fastify routes in apps/api

[collinsezedike]

Summary

apps/api has zero HTTP-layer tests. The Fastify route handlers (positions, deposit, withdraw, health) are only exercised end-to-end against a live network. Unit tests exist for SDK helpers, but nothing verifies that the route layer parses requests correctly, validates inputs, serialises responses, or returns the right HTTP status codes.

Motivation

Without HTTP-layer tests, a change to a route handler (wrong status code on error, missing field in response, broken input validation) ships silently. Fastify's inject API makes it possible to test routes without a real network -- the suite can run in CI with mocked SDK helpers.

Proposed Solution

Add a Vitest test file at apps/api/src/__tests__/routes.test.ts using fastify.inject to test each route:

• GET /health -- 200 with expected body shape
• GET /positions -- 200 with mocked SDK response; 400 on missing wallet param; 500 when SDK throws
• POST /deposit -- 200 with mocked prepareSorobanTx; 400 on invalid body; 422 on simulation failure
• POST /withdraw -- same shape as deposit

Mock @meridian/stellar-sdk-helpers at the module level so no Soroban RPC calls are made.

Scope

Field	Value
Area	api, testing
Protocol affected	none (mocked)
Network	n/a
Breaking change?	No

Acceptance Criteria

• apps/api/src/__tests__/routes.test.ts covers all four route groups
• Tests run without a live RPC endpoint
• pnpm test passes from the repo root