Add test infrastructure with fixtures, helpers, and integration tests

- Add [lib] target to enable integration test imports
- Add rcgen and reqwest dev-dependencies for TLS and HTTP test helpers
- Create src/config/test_fixtures.rs with test_static_config() and test_dynamic_config()
- Create tests/ with integration tests, HTTP test helper (TestUpstream), and TLS test helper (SelfSignedCert)
- Add Clone derives to StaticConfig and related structs for test fixture construction
- All existing tests continue to pass

src/main.rs

@@ -1,12 +1,3 @@
-mod admin;
-mod config;
-mod health;
-mod logging;
-mod proxy;
-mod rate_limit;
-mod shutdown;
-mod tls;
-
 fn main() {
     tracing::info!("reverse-proxy starting");
 }