Add Lua unit testing infrastructure with busted

[Copilot]

Implements comprehensive unit testing for Lua configuration modules using busted framework, integrated with Nix build system.

Implementation

Test Suite (tests/)

• helpers_spec.lua - Tests git_root() with edge cases (nil handles, empty results, whitespace trimming)
• health_spec.lua - Tests health check system (program/LSP registration, check execution)
• lazy_spec.lua - Tests lazy loading system (spec management, plugin loading)
• example_spec.lua - Template demonstrating BDD patterns and vim API mocking

Integration

• Added tests/default.nix as flake-parts module exposing packages.lua-tests and checks.lua-tests
• Added busted to dev shell packages
• Configured via .busted to exclude example template from test runs

Tooling

• run-tests.sh - Convenience wrapper checking busted availability
• tests/README.md - Usage documentation

Usage

# Development iteration
nix develop
busted tests/

# CI/production
nix build .#lua-tests
nix flake check

Tests mock vim APIs since they run outside Neovim runtime:

_G.vim = {
  fn = { executable = function() return 1 end },
  api = { nvim_create_autocmd = function() end },
  lsp = { config = {} }
}

Updated copilot instructions and validation steps to include test execution.

Original prompt

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This section details on the original issue you should resolve

<issue_title>lua unit-tests (optional, but highly recommended)</issue_title>
<issue_description>Provide lua unit tests</issue_description>

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

• Fixes lua unit-tests (optional, but highly recommended) #95

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