fix: llvm-cov compilation failure: gate schemars usage behind feature flag (#1064)

* Initial plan

* Fix code coverage compilation: gate schemars usage behind cfg(feature = "schemars")

Agent-Logs-Url: https://github.com/iotaledger/iota-rust-sdk/sessions/e6987052-c285-4472-a023-853238f65337

Co-authored-by: Thoralf-M <46689931+Thoralf-M@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Thoralf-M <46689931+Thoralf-M@users.noreply.github.com>

Author: Copilot

crates/iota-sdk-types/src/serialization_proptests.rs

@@ -9,6 +9,7 @@ use crate::*;
 macro_rules! serialization_test {
     ($type:ident) => {
         paste::item! {
+            #[cfg(feature = "schemars")]
             #[cfg_attr(target_arch = "wasm32", proptest(cases = 50))]
             #[cfg_attr(not(target_arch = "wasm32"), proptest)]
             #[allow(non_snake_case)]
@@ -35,6 +36,7 @@ macro_rules! serialization_test {
     };
 }
 
+#[cfg(feature = "schemars")]
 fn assert_valid_json_schema<T>(instance: &T)
 where
     T: serde::Serialize + schemars::JsonSchema,