fix(fuzz): explicit HostError generic on try_invoke_contract

`soroban-sdk` 22.x changed `Env::try_invoke_contract` to return
`Result<Result<T, ContractError>, HostError>`. The fuzz harnesses
were calling it as `env.try_invoke_contract::<T, _>(...)`, leaving
the host-error type uninferred; with multiple `impl TryFromVal<...>`
candidates for `Error`, rustc refused with:

  error[E0283]: type annotations needed
    --> fuzz_targets/fuzz_vault_withdraw.rs:42:13
       |
    42 |     let _ = env.try_invoke_contract::<(), _>(
       |                  ^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the error type `E`

In the looping harness, the same problem cascaded — the
single `.expect()` was returning `Result<u64, Error>` (the inner
layer) instead of `u64`, breaking the `position_id >= 1` and
`{position_id}` formatting assertions.

Fix: make the second generic argument explicit as
`soroban_sdk::Error` (which is the actual host-error type for the
outer Result), and where `.expect()` is also used, unwrap both layers:

  // vault/deposit, vault/withdraw
  - try_invoke_contract::<(), _>
  + try_invoke_contract::<(), soroban_sdk::Error>

  // looping/open_position
  - try_invoke_contract::<u64, _>
  + try_invoke_contract::<u64, soroban_sdk::Error>
  - let position_id = result.expect("...");
  + let position_id = result
  +     .expect("open_position returned a host error")
  +     .expect("open_position with valid args returned a contract error");

The rewards harness only uses `env.invoke_contract` (infallible) and
needed no change.

Author: LaGodxy

quantara/soroban/contracts/looping/fuzz/fuzz_targets/fuzz_looping_open_position.rs

@@ -27,7 +27,9 @@ fuzz_target!(|data: &[u8]| {
     let contract_id = env.register(LoopingContract, ());
     let user = Address::generate(&env);
 
-    let result = env.try_invoke_contract::<u64, _>(
+    // try_invoke_contract returns `Result<Result<u64, ContractError>, HostError>`;
+    // explicitly specify the host error type so type inference succeeds.
+    let result = env.try_invoke_contract::<u64, soroban_sdk::Error>(
         &contract_id,
         &Symbol::new(&env, "open_position"),
         soroban_sdk::vec![
@@ -39,7 +41,10 @@ fuzz_target!(|data: &[u8]| {
     );
 
     if collateral > 0 && (100..=500).contains(&leverage) {
-        let position_id = result.expect("open_position with valid args returned error");
+        // Unwrap the outer (host) Err first, then the inner (contract) Err.
+        let position_id = result
+            .expect("open_position returned a host error")
+            .expect("open_position with valid args returned a contract error");
         assert!(position_id >= 1, "position_id must be >= 1, got {position_id}");
     }
     // Invalid inputs may error; no further assertion needed.

quantara/soroban/contracts/vault/fuzz/fuzz_targets/fuzz_vault_deposit.rs

@@ -32,7 +32,9 @@ fuzz_target!(|data: &[u8]| {
     let user = Address::generate(&env);
 
     // Invoke deposit directly via the registered contract.
-    let result = env.try_invoke_contract::<(), _>(
+    // try_invoke_contract returns `Result<Result<(), ContractError>, HostError>`;
+    // specify the host error type as soroban_sdk::Error so type inference can succeed.
+    let result = env.try_invoke_contract::<(), soroban_sdk::Error>(
         &contract_id,
         &soroban_sdk::symbol_short!("deposit"),
         soroban_sdk::vec![&env, user.to_val(), amount.into_val(&env)],

quantara/soroban/contracts/vault/fuzz/fuzz_targets/fuzz_vault_withdraw.rs

@@ -38,8 +38,10 @@ fuzz_target!(|data: &[u8]| {
         );
     }
 
-    // Attempt withdrawal.
-    let _ = env.try_invoke_contract::<(), _>(
+    // Attempt withdrawal. try_invoke_contract returns
+    // `Result<Result<(), ContractError>, HostError>`; specify the host error
+    // type to enable type inference (otherwise rustc can't pick `E`).
+    let _ = env.try_invoke_contract::<(), soroban_sdk::Error>(
         &contract_id,
         &soroban_sdk::symbol_short!("withdraw"),
         soroban_sdk::vec![&env, user.to_val(), withdraw_amount.into_val(&env)],