Introduced reusable runtimes for runtime tests

[tofarr]

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End-user friendly description of the problem this fixes or functionality this introduces.
This PR introduces reusable runtime fixtures for tests that don't modify the container state, significantly improving test performance by reducing the number of runtime environments that need to be created and torn down during testing.

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Summarize what the PR does, explaining any non-trivial design decisions.

1. Introduces module-scoped reusable runtime fixtures:

   • reusable_runtime_and_config - Creates a shared runtime that persists across multiple tests
   • reusable_runtime and reusable_config - Convenience fixtures that extract the runtime and config

2. Improves runtime management:

   • Renamed _load_runtime to create_runtime_and_config for better clarity
   • Enhanced close_test_runtime to properly clean up resources
   • Added helper functions for common operations

3. Optimizes test execution:

   • Tests that don't modify container state now share a runtime instance
   • Tests that need isolation still create their own runtime
   • Significantly reduces test execution time, especially for Docker-based tests

These changes maintain test isolation where needed while improving performance by reusing runtimes when possible. This is particularly valuable for CI/CD pipelines and local development.

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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:5554cad-nikolaik   --name openhands-app-5554cad   docker.all-hands.dev/all-hands-ai/openhands:5554cad

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