[Test Improver] Add unit tests for commands/_helpers.py I/O and config helpers

[danielmeppiel]

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Goal and Rationale

src/apm_cli/commands/_helpers.py is a shared utility module used by every command (init, install, run, list, config, etc.). It was at 59% coverage despite containing critical I/O helpers. This PR adds targeted tests for the highest-value uncovered paths.

What's Tested

Function	What's Covered
_atomic_write	Normal write, overwrite, empty string, unicode, temp-file cleanup on error
_update_gitignore_for_apm_modules	Creates file when absent, skips when entry exists, appends to existing, adds comment header, handles read error
_load_apm_config	Returns None when no apm.yml, parses config, handles scripts section
_get_default_script	Returns None when missing, returns "start" when present
_list_available_scripts	Returns {} when no file/scripts, returns full scripts dict
_scan_installed_packages	Dir absent, 2-level packages, 3-level packages, dot-named dirs, no-marker dirs
_check_and_notify_updates	E2E env var skip, unknown version skip, no-op when up-to-date, warns with version, silently ignores exceptions

Coverage Impact

File	Before	After	Δ
commands/_helpers.py	59%	80%	+21pp
Total	58%	58%	0pp

Approach

• Pure unit tests using tmp_path, monkeypatch, and unittest.mock.patch
• No new dependencies
• Each class tests one function; each test one behavior

Test Status

30 passed in 0.27s
1938 passed in 14.03s  # full suite

Commands

# Run new tests only
uv run pytest tests/unit/test_command_helpers.py -v

# Full suite
uv run pytest tests/unit/ --no-header -q

# With coverage
uv run pytest tests/unit/ --cov=apm_cli --cov-report=term-missing

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