fix: resolve CI test failures

[AhmedKorim]

Summary

• Fixed missing run_node and shutdown_event function exports that were causing test collection failures
• Updated test imports to use available functions instead of non-existent ones
• Applied code formatting fixes with black
• Partially addressed linting issues with ruff

Root Cause

The CI was failing because:

1. src/node/__init__.py was exporting functions that didn't exist (run_node, shutdown_event)
2. tests/test_node.py was trying to import these non-existent functions
3. This caused pytest to fail during test collection, not during test execution

Changes Made

• Removed non-existent functions from __all__ in src/node/__init__.py
• Updated tests/test_node.py to test the actual main function that exists
• Applied automatic code formatting with black
• Fixed some linting issues with ruff

Test Plan

• Tests can now be collected without import errors
• Test execution works (9/10 tests pass, 1 NATS-related test fails due to test isolation)
• Code formatting passes
• Docker build still fails due to repository structure mismatch (separate issue)

🤖 Generated with Claude Code