fix: properly expand tilde (~) in path configuration options

[yuyuan12138]

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Title

fix: properly expand tilde (~) in path configuration options

Description

This PR fixes the path resolution bug reported in #78, where configuration options that accept file paths would not properly expand the tilde (~) to the user's home directory.

Problem

Previously, when users configured paths like:
testcases_directory = "~/cp/testcases"

The plugin would incorrectly treat this as a relative path, creating a directory literally named ~/cp/testcases in the current working directory, instead of resolving it to
/home/user/cp/testcases.

Solution

This fix introduces proper path expansion and normalization:

1. New utility functions in utils.lua:
   - normalize_path() - Expands ~ to home directory and converts to absolute path
   - resolve_config_path() - Smart resolution that distinguishes between absolute paths (starting with ~ or /) and relative paths
2. Path normalization during config loading:
   - Added normalize_path_config() in config.lua to normalize path options when configuration is loaded
   - Applied to both global setup and local .competitest.lua configurations
3. Updated all path-related configuration usage:
   - testcases_directory
   - compile_directory
   - running_directory
   - received_problems_path
   - received_contests_directory
   - received_contests_problems_path
   - template_file

Backward Compatibility

✅ Fully backward compatible - all existing configurations continue to work:

• Relative paths (., ./path, ../path) work as before
• $(HOME) modifier in file format strings continues to work
• Absolute paths work correctly

Example Usage

After this fix, all of these configurations work correctly:

require('competitest').setup({
-- Tilde expansion (newly fixed)
testcases_directory = "~/cp/testcases",

-- Absolute paths (already worked)
testcases_directory = "/home/user/cp/testcases",

-- Relative paths (still work)
testcases_directory = "./testcases",
testcases_directory = ".",

-- HOME modifier (still works)
testcases_directory = "$(HOME)/cp/testcases",

})

Changes

• lua/competitest/utils.lua - Add path normalization functions
• lua/competitest/config.lua - Normalize paths during config loading
• lua/competitest/testcases.lua - Use smart path resolution
• lua/competitest/runner.lua - Use smart path resolution
• lua/competitest/receive.lua - Use smart path resolution

Total: 5 files changed, 85 insertions(+), 4 deletions(-)

Testing

Thoroughly tested with:

• ✅ Tilde expansion (~/path)
• ✅ Absolute paths (/absolute/path)
• ✅ Relative paths (., ./path, ../path)
• ✅ $(HOME) modifier compatibility
• ✅ Configuration hierarchy (global/local configs)
• ✅ All path-related configuration options

Fixes #78

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