Add unit and integration tests for the kernels API

[Zsailer]

Summary

Added comprehensive unit and integration tests for the kernels API, including complex document-aware kernel
client testing that verifies YDoc message tracing and output handling. Also includes the previous kernel restart
fix.

Changes

Test Infrastructure

• Created comprehensive test suite in jupyter_server_documents/tests/kernels/ with 62 tests
• Added proper async test configuration with pytest-asyncio

Test Coverage

• Kernel Manager Tests (8 tests) - State management, lifecycle transitions, restart functionality
• Kernel Client Tests (9 tests) - Basic client functionality, listener management, YRoom integration
• Multi-Kernel Manager Tests (7 tests) - Multi-kernel management and restart operations
• States Tests (15 tests) - Enum functionality for lifecycle and execution states
• WebSocket Connection Tests (7 tests) - Message serialization, error handling, protocol validation
• Integration Tests (11 tests) - Complex end-to-end document-aware kernel client workflows

Key Integration Test Features

The integration tests simulate real kernel execution scenarios (like 2 + 2) and verify:

• Execute input messages properly update execution counts in YDoc
• Status messages correctly update cell execution states (busy → running, idle)
• Output messages (execute_result, stream, error, display_data) are processed and stored
• Complete execution flows work end-to-end: execute_input → status → output → status
• Multiple cells can maintain different execution states simultaneously
• Kernel-level status updates propagate to awareness state
• Message processing without cell_id skips cell-specific updates

YDoc API Enhancements

• Added get_cell_list() method to YNotebook for accessing all cells
• Added get_meta() method to YNotebook for accessing notebook metadata
• Fixed integration tests to use local extended YNotebook implementation

[ellisonbg]

@Zsailer can you commit your changes to pyproject.toml?

[Zsailer]

Actually, it doesn't look like a change to the pyproject.toml is needed after all. The dependency I thought I needed was already there. I'll remove this line from the top level comment.

[Zsailer]

Closing in favor of #146