fix: deflake //rs/tests/idx:ii_delegation_test (#9796)

## Root Cause

The `update()` method in `AgentWithDelegation` is fire-and-forget — it
sends the HTTP call but doesn't poll for execution through consensus.
The test then asserts the counter was incremented using
`assert_canister_counter_with_retries`, but with only 10 retries × 1s =
10 seconds total wait time, the update may not have been processed
through consensus yet.

In the observed failure on 2026-04-08, the update was sent at 15:25:50
and the assertion timed out at 15:26:01 (~11 seconds), just barely
exceeding the 10-second retry window.

## Fix

Increased retry parameters from 10 retries × 1s (10s total) to 30
retries × 2s (60s total), giving ample time for the update to go through
consensus.

Verified with `--runs_per_test=3 --jobs=3` — all 3 runs passed (avg
67.9s).

---

This PR was created following the steps in
`.claude/skills/fix-flaky-tests/SKILL.md`.

Author: basvandijk

rs/tests/idx/ii_delegation_test.rs

@@ -134,8 +134,8 @@ pub fn test(env: TestEnv) {
         &counter_canister_id,
         vec![],
         1,
-        10,
-        Duration::from_secs(1),
+        30,
+        Duration::from_secs(2),
     ));
     let expected_principal = Principal::self_authenticating(&ii_derived_public_key);
     info!(