fix: migrate to AsyncIterableProducer in terminal shell integration to avoid memory leaks

[Diode11-Alt]

Summary

This PR migrates AsyncIterableObject usage in extHostTerminalShellIntegration.ts to AsyncIterableProducer.

Rationale

AsyncIterableObject buffers all emitted values indefinitely, which leads to memory leaks in single-consumption scenarios like terminal shell execution data streams. AsyncIterableProducer is a more memory-efficient alternative that doesn't buffer values.

Changes

• Replaced AsyncIterableObject with AsyncIterableProducer in ShellExecutionDataStream.
• Updated InternalTerminalShellExecution._createDataStream to use AsyncIterableProducer.EMPTY.
• Refactored ShellExecutionDataStream.flush() to wait for the completion barrier instead of draining the iterable with .toPromise().

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