hitl_tool: _poll_for_decision ignores interval parameter and always sleeps for POLL_INTERVAL_S #191

Describe the bug
hitl_tool accepts a poll_interval: float = POLL_INTERVAL_S argument and forwards it as interval to _poll_for_decision(api_base, request_id, poll_interval, timeout) in src/google/adk_community/tools/hitl/gateway.py. However, inside _poll_for_decision, the sleep call on each poll cycle ignores the interval parameter and hardcodes the module-level POLL_INTERVAL_S constant:

await asyncio.sleep(POLL_INTERVAL_S + random.uniform(0, POLL_JITTER_S))

As a result, passing a custom poll_interval to @hitl_tool(agent_name="...", poll_interval=0.25) (or calling _poll_for_decision(..., interval=0.25)) has no effect and always sleeps for POLL_INTERVAL_S (2.0s) plus jitter.

To Reproduce

1. Decorate a tool with a custom poll_interval, e.g., @hitl_tool(agent_name="credit_agent", poll_interval=0.25).
2. Have GET /approvals/{id} return {"status": "pending"} on the first poll before transitioning to {"status": "approved"}.
3. Observe that _poll_for_decision sleeps for POLL_INTERVAL_S + random.uniform(0, POLL_JITTER_S) instead of interval + random.uniform(0, POLL_JITTER_S).

Expected behavior
_poll_for_decision should use its interval parameter (await asyncio.sleep(interval + random.uniform(0, POLL_JITTER_S))), while still defaulting to POLL_INTERVAL_S when poll_interval is not explicitly passed to hitl_tool.

Proposed fix
Update _poll_for_decision in src/google/adk_community/tools/hitl/gateway.py to sleep for interval + random.uniform(0, POLL_JITTER_S) and add unit tests in tests/unittests/tools/test_hitl_gateway.py covering custom poll_interval sleep behavior across pending polls and timeout handling.