Fix constant error observations appearing in the logs

[tofarr]

Now checking that .gitignore exists. before trying to filter based on its contents.

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I see the following constantly in the logs:

|21:13:47 - openhands:DEBUG: action_execution_server.py:166 - Running action:
    |FileReadAction(path='.gitignore', start=0, end=-1, thought='', action='read', security_risk=None)
    ...
    |21:13:47 - openhands:DEBUG: action_execution_server.py:168 - Action output:
    |**ErrorObservation**
    |File not found: /workspace/.gitignore. Your current working directory is /workspace.
    ...

The reason is that every time we call /list-files we try to filter based on the contents of .gitignore - so we get an error message every time we try to list files if there is no .gitignore.

This change checks that the .gitignore exists before trying to read it.

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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:6e252fe-nikolaik   --name openhands-app-6e252fe   docker.all-hands.dev/all-hands-ai/openhands:6e252fe

[enyst]

For some reason, one of the image builds got stuck for over 4h. I restarted them.

[tofarr]

I'm gonna punt on this one now as there does seem to be concerns about the additional overhead of a file existence check. Perhaps a more thorough refactor will allow a better solution