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/lib.rs

@@ -0,0 +1,8 @@
+pub mod admin;
+pub mod config;
+pub mod health;
+pub mod logging;
+pub mod proxy;
+pub mod rate_limit;
+pub mod shutdown;
+pub mod tls;