fix: correct apply_delta_to_u128 signature to take &i128 (closes #407)

The IDataStore trait declared delta: i128 (owned) while every call site
in the same file passes a reference (&fee_tokens, &net_collateral).
Changed the trait declaration to delta: &i128 to match the Soroban client
convention and all existing call sites.

Author: Obiajulu-gif

libs/increase_position_utils/src/lib.rs

@@ -28,7 +28,7 @@ trait IDataStore {
     fn get_u128(env: Env, key: BytesN<32>) -> u128;
     fn get_i128(env: Env, key: BytesN<32>) -> i128;
     fn set_u128(env: Env, caller: Address, key: BytesN<32>, value: u128) -> u128;
-    fn apply_delta_to_u128(env: Env, caller: Address, key: BytesN<32>, delta: i128) -> u128;
+    fn apply_delta_to_u128(env: Env, caller: Address, key: BytesN<32>, delta: &i128) -> u128;
     fn apply_delta_to_i128(env: Env, caller: Address, key: BytesN<32>, delta: i128) -> i128;
     fn get_address(env: Env, key: BytesN<32>) -> Option<Address>;
     fn add_bytes32_to_set(env: Env, caller: Address, set_key: BytesN<32>, value: BytesN<32>);