Add integration tests for Neovim configuration

[Copilot]

Adds headless integration tests to verify configuration loads correctly and core functionality works.

Test Infrastructure

• tests/default.nix - Three test derivations exposed via flake checks:

  • nvim-config-loads - Smoke test for error-free loading
  • nvim-health-check - Validates :checkhealth nobbz output
  • nvim-lua-tests - 7 test cases covering LSP, lazy loading, and plugin initialization

• tests/test-runner.lua - Minimal test framework with pcall-based test execution and explicit exit codes

• tests/README.md - Test structure and usage documentation

Integration

# Run all tests
nix flake check

# Run individual test
nix build .#checks.x86_64-linux.nvim-lua-tests

Tests run in headless mode with temporary $HOME, validating:

• Configuration loading without errors
• Health check module availability
• LSP configuration registration via vim.lsp.config table iteration
• Lazy loading system (nobbz.lazy) functionality
• Core plugin loading (blink.cmp, telescope)

Documentation

• Updated README.md with testing section in update process
• Updated .github/copilot-instructions.md with test commands and structure
• Reordered validation steps to prioritize nix flake check

Implementation Notes

• 250ms defer timeout for configuration loading (increased from initial 100ms)
• Uses os.exit() for explicit exit codes rather than vim commands
• Test runner loaded via dofile() to avoid runtimepath manipulation

Original prompt

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This section details on the original issue you should resolve

<issue_title>nvim integration tests (mandatory)</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes nvim integration tests (mandatory) #96

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