PR-Codex overview

This PR focuses on updating the codebase by replacing the cancelled() method with the done() method in multiple files.

Detailed summary

• Replaced cancelled() with done() in kernel_client_tests.py
• Replaced cancelled() with done() in kernel/kernel_client.py
• Replaced cancelled() with done() in base_agent.py
• Replaced cancelled() with done() in agent_server.py
• Replaced cancelled() with done() in kernel_rpc_server.py
• Replaced cancelled() with done() in openai_agent.py

The following files were skipped due to too many changes: up/src/octopus_up/up.py

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