Fix file descriptor leak in S3FileStore

[rbren]

This PR fixes a potential file descriptor leak in the S3FileStore implementation. The issue was in the read() method where the StreamingBody object from boto3 was not being properly closed after use.

Changes

• Modified read() method to use a context manager (with statement) to ensure proper cleanup of the StreamingBody object

Why

• The StreamingBody object from boto3 maintains an open HTTP connection to S3
• Without proper closing, these connections can accumulate over time
• This can lead to resource exhaustion, particularly in long-running applications or those that read many files

The fix ensures that the stream is properly closed even if an error occurs during reading, preventing file descriptor leaks.

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:30b5056-nikolaik   --name openhands-app-30b5056   docker.all-hands.dev/all-hands-ai/openhands:30b5056