fix(tests): resolve all clippy warnings for CI compliance

- Remove needless borrows on format!() in set_body_string() calls (needless_borrows_for_generic_args)
- Replace assert!(false, ...) with collected assertion (assertions_on_constants + never_loop)
- Use direct Method::POST comparison instead of to_string() (cmp_owned)
- Simplify negated equality to != operator (nonminimal_bool)

CI pipeline now passes with -D warnings enabled

tests/unit/enroll_identity.rs

@@ -245,7 +245,7 @@ fn test_ip_addresses_are_unicast() {
 
         // Not unspecified (0.0.0.0)
         assert!(
-            !(parts == vec![0, 0, 0, 0]),
+            parts != vec![0, 0, 0, 0],
             "Address '{}' is unspecified",
             addr
         );
@@ -492,7 +492,7 @@ fn test_cross_distro_os_release_parsing() {
         parsed.contains_key("NAME"),
         "os-release must contain NAME field"
     );
-    assert!(parsed["NAME"].ne(&""), "NAME should not be empty");
+    assert!(!parsed["NAME"].is_empty(), "NAME should not be empty");
 }
 
 #[test]