[Bug]: Testing library waits extra time before completing test

### Expected Behavior

The testing library test exits immediately when the execution finishes.

### Actual Behavior

Every execution that runs in the testing library waits an additional 100ms for the test to drain its checkpoint queue. This is due to a hack where we wait an extra 100ms before exiting the `run` function.

See: https://github.com/aws/aws-durable-execution-sdk-js/blob/main/packages/aws-durable-execution-sdk-js-testing/src/test-runner/local/test-execution-orchestrator.ts#L166-L171

### Steps to Reproduce

1. Run an execution and log the time at the end
2. The promise will resolve 100ms after the end of the execution

### SDK Version

1.1.0

### Node.js Version

22.x

### Is this a regression?

No

### Last Working Version

_No response_

### Additional Context

_No response_

Start in packages/aws-durable-execution-sdk-js-testing/src/test-runner/local/test-execution-orchestrator.ts at lines 166-171, where the extra delay before run completes is described. Trace the execution completion path and verify that the promise resolves immediately after execution finishes, without the additional 100ms checkpoint-queue wait.

Written by the indexing model from the issue text.