test: add coverage for config_manager and env_checker

[bigcat88]

Both config_manager.py and env_checker.py had zero unit test coverage. This adds 58 tests across two new files covering all public methods.

test_config_manager.py (36 tests) covers load() with background parsing and stale PID cleanup, write_config() directory creation, set()/get() round-trips, get_bool() with all accepted formats, get_or_override() priority (CLI > env > config), get_env_data() output for all config combinations, and remove_background().

test_env_checker.py (22 tests) covers format_python_version() old/modern boundary, check_comfy_server_running() with mocked requests, and EnvChecker methods including check() with virtualenv/conda detection, is_isolated_env(), get_isolated_env(), and fill_print_table().

Both modules use the @singleton decorator, so tests extract the original class from the wrapper closure to construct isolated instances backed by tmp_path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #390      +/-   ##
==========================================
+ Coverage   70.98%   72.59%   +1.60%     
==========================================
  Files          33       33              
  Lines        3915     3915              
==========================================
+ Hits         2779     2842      +63     
+ Misses       1136     1073      -63     

see 2 files with indirect coverage changes

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