Fix CLI /new command not properly resetting conversation display

[xingyaoww]

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End-user friendly description of the problem this fixes or functionality this introduces.

Fixed an issue where the CLI /new command would show old session statistics before starting a new conversation. Now when users type /new, the screen is cleared immediately for a cleaner transition to the new session.

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

This PR fixes issue #11123 where the CLI /new command wasn't properly resetting the conversation display. The problem was that when users requested a new session, the old session's statistics (usage metrics, conversation history) were displayed before the new session started, creating a confusing user experience.

Changes made:

1. Modified handle_new_command() in openhands/cli/commands.py: Replaced the call to display_shutdown_message(usage_metrics, sid) with clear() from prompt_toolkit.shortcuts. This provides a cleaner transition by immediately clearing the screen instead of showing old session stats.

2. Added comprehensive tests in tests/unit/cli/test_new_command_reset.py to verify:

   • Session IDs are properly generated as unique values
   • Usage metrics are reset for new sessions
   • The /new command behavior correctly clears the screen instead of displaying shutdown messages

Design decision rationale:
The core session mechanics (unique session ID generation, fresh UsageMetrics objects, proper state reset) were already working correctly. The issue was purely a user experience problem where old session information was displayed during the transition. By clearing the screen immediately, users get a clean slate when they explicitly request a new session with /new.

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Link of any specific issues this addresses:

Fixes #11123

@xingyaoww can click here to continue refining the PR

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To run this PR locally, use the following command:

GUI with Docker:

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:0afbe22-nikolaik   --name openhands-app-0afbe22   docker.all-hands.dev/all-hands-ai/openhands:0afbe22

CLI with uvx:

uvx --python 3.12 --from git+https://github.com/All-Hands-AI/OpenHands@openhands-workspace-kxha57nv openhands

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

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If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #11125 at branch `openhands-workspace-kxha57nv`

Feel free to include any additional details that might help me get this PR into a better state.

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