[improvement]: add logic to detect thread pool leak

[zhongkechen]

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Issue Link, if available

Fixes #297

Description

• When an invocation completes, validate that the user thread pool is empty
• In local test runner, add nextAttemptDelaySeconds for steps in READY/PENDING status to align with the backend behavior
• If a user thread is stuck running, SDK will always wait for it to complete before moving on to the next lambda request in order to prevent thread pool leaks. The execution will fail in this case, so it would be easier for users to detect the stuck thread issue.

Demo/Screenshots

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