test(server): add integration test suite for pipeline, HTTP, and interceptors (#116)

Expose test_helpers module via `test-helpers` feature flag for external
integration tests. Add 3 test files:

- pipeline_integration_tests.rs (17 tests): end-to-end pipeline flow
  with schema validation, skip_validation, database override,
  interceptor chains, executor errors, validate-only endpoint
- http_integration_tests.rs (18 tests): full HTTP request/response via
  axum tower::ServiceExt oneshot — health, schema, query success/error,
  validation, interceptor via HTTP, routing (404/405)
- interceptor_integration_tests.rs (10 tests): multi-interceptor chain
  ordering (request forward, response reverse), rejection short-circuit,
  metadata propagation, audit log file output through full pipeline

Author: ds1sqe