Add bounds check for heartbeat interval

[yuandrew]

What was changed

Only allow worker heartbeating interval to be between 1s and 60s

Also changed heartbeat test to account for if host_cpu_usage is so low it rounds down to 0.0.

Why?

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

---

Note

Adds builder validation to enforce heartbeat_interval between 1s and 60s and updates integration tests to use 1s heartbeats with adjusted timings/assertions.

• Core:
  • RuntimeOptions now uses builder validation to enforce heartbeat_interval in [1s, 60s] and derives Default (default remains Some(60s)) in crates/sdk-core/src/lib.rs.
• Tests:
  • Update integration tests in crates/sdk-core/tests/integ_tests/worker_heartbeat_tests.rs to use heartbeat_interval(Some(Duration::from_secs(1))) instead of 100ms.
  • Increase sleeps/time thresholds (e.g., 150ms→1500ms, 200ms→2000ms, sleep 200ms→2s) and bump some activity start_to_close_timeout values (1s→5s).
  • Relax host usage assertions from nonzero to >= 0.

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[cursor]

Bug: Test assertions weakened for CPU and memory usage

The assertions for current_host_cpu_usage and current_host_mem_usage were changed from verifying non-zero values to only checking non-negative values. This weakens the test significantly because it no longer validates that host metrics are actually being collected and reported, only that they aren't negative, which provides minimal value and could allow bugs in metrics collection to go undetected.