Write a comprehensive integration test suite for all HTTP endpoints

[mftee]

Write a comprehensive integration test suite for all HTTP endpoints in the Rust service. Tests must spin up a real Axum server and make actual HTTP requests using a test client, with mocked Stellar and Redis backends.

Requirements:

• Use axum-test or tower::ServiceExt to create a test router without binding to a port
• Create test fixtures and helpers:
  • test_app() — builds the full Axum router with mocked StellarClient and in-memory state
  • mock_stellar_verify(hash, verified) — configures the mock to return a given result
  • valid_sha256() — helper that returns a random valid SHA-256 hex string
• Write integration tests for every endpoint:
  • GET /health → 200
  • GET /metrics → 200 with Prometheus content
  • POST /verify — valid hash → 200; invalid hash → 400; Stellar failure → 500
  • GET /verify/:hash — same as above via GET
  • POST /verify/batch — valid batch; too many hashes → 400; empty → 400
  • GET /verify/:hash/history — found; not found → 404
  • POST /submit — valid submission; missing secret key → appropriate error
  • POST /revoke — existing hash; non-existent hash → 404
  • POST /transfer — valid; invalid date → 400
• All tests must be independent (no shared state between tests)

Acceptance Criteria:

• All endpoint tests pass with cargo test
• No tests share mutable state
• Test coverage includes both happy path and error cases for every endpoint
• Tests run without any external services (no real Redis, no real Stellar)