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

tests/helpers/tls_test_helper.rs

@@ -0,0 +1,28 @@
+use rcgen::{CertificateParams, DistinguishedName, KeyPair};
+
+pub struct SelfSignedCert {
+    pub cert_pem: String,
+    pub key_pem: String,
+}
+
+pub fn generate_self_signed_cert(domains: &[&str]) -> SelfSignedCert {
+    let mut params = CertificateParams::new(
+        domains
+            .iter()
+            .map(|s| s.to_string())
+            .collect::<Vec<String>>(),
+    )
+    .unwrap();
+    params.distinguished_name = DistinguishedName::new();
+    params
+        .distinguished_name
+        .push(rcgen::DnType::CommonName, "test.local");
+
+    let key_pair = KeyPair::generate().unwrap();
+    let cert = params.self_signed(&key_pair).unwrap();
+
+    SelfSignedCert {
+        cert_pem: cert.pem(),
+        key_pem: key_pair.serialize_pem(),
+    }
+}