test: add comprehensive tests for user-defined template variables

[paris3200]

Summary

Adds comprehensive test coverage for user-defined template variables to ensure the extensible template system works correctly.

Changes

• Config test: Verifies user template variables merge properly with defaults
• Integration test: Ensures config-defined template variables work in actual substitution
• Override test: Confirms custom variables can override config variables
• Coverage: Tests both function and static value template variables

Test Cases Added

1. merges user-defined template variables with defaults - Tests that user variables are properly merged while preserving defaults
2. uses config-defined template variables - Integration test that config variables work in template substitution
3. overrides config variables with custom vars parameter - Tests precedence of custom vars over config vars

Why This Matters

This ensures the plugin's extensible template system works as documented, giving users confidence they can add custom variables like:

require("markdown-notes").setup({
  template_vars = {
    author = function() return "Your Name" end,
    project = function() return vim.fn.getcwd():match("([^/]+)$") end,
  },
})

All tests pass ✅