Small rename to long term memory

[enyst]

Give a summary of what the PR does, explaining any non-trivial design decisions

This PR proposes to rename long term memory to long_term_memory.py.

It's just a small thing, the memory module contains currently condensers (in use) and long term memory (not in use), and it frees us to use the name.

This also makes the diff of PR 6909 easier to read, because git diff gets confused since this rename is there too.

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Link of any specific issues this addresses

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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:77e113b-nikolaik   --name openhands-app-77e113b   docker.all-hands.dev/all-hands-ai/openhands:77e113b