Add sandbox functionality

[amyguan3]

Add route for agent to attach sandbox / upload file to the sandbox. Records the sandbox ID in agent metadata (sandbox_id) for retrieval during tool use.

Example testing/usage:

echo "Attaching sandbox to agent $AGENT_ID..."
curl -X PATCH "http://localhost:8283/v1/agents/$AGENT_ID/sandbox/attach" \
    -H 'Content-Type: application/json'

echo -e "\n\nUploading test.txt to sandbox..."
curl -X POST "http://localhost:8283/v1/agents/$AGENT_ID/sandbox/upload" \
    -F "[email protected]" \
    -F "path=/home/user/new_test.txt"

Add built in tool (server side) to run bash commands called run_bash that agent can call to run code inside its designated sandbox.