Fix MLflow CWD-sensitive workflow paths

[Kevin-Li-2025]

Summary

• resolve file:// MLflow tracking URIs with url2pathname before building the experiment FileLock path, so absolute tracking paths stay absolute regardless of the caller CWD
• skip optional uncommitted-code artifact logging when the process CWD is not a git work tree
• remove shell=True from the git diff/status calls and capture stderr so git diagnostics do not leak into caller stderr
• add regression coverage for absolute file URI lock paths and quiet non-git CWD handling

Fixes #2252.

Test plan

• MLFLOW_ALLOW_FILE_STORE=true .venv/bin/python -m pytest tests/dependency_tests/test_mlflow.py -q
• .venv/bin/python -m black qlib/workflow/expm.py qlib/workflow/recorder.py tests/dependency_tests/test_mlflow.py -l 120 --check --diff
• .venv/bin/python -m pylint --disable=C0104,C0114,C0115,C0116,C0301,C0302,C0411,C0413,C1802,R0401,R0801,R0902,R0903,R0911,R0912,R0913,R0914,R0915,R0917,R1720,W0105,W0123,W0201,W0511,W0613,W1113,W1514,W4904,E0401,E1121,C0103,C0209,R0402,R1705,R1710,R1725,R1730,R1735,W0102,W0212,W0221,W0223,W0231,W0237,W0612,W0621,W0622,W0703,W1309,E1102,E1136 --const-rgx='[a-z_][a-z0-9_]{2,30}' qlib/workflow/expm.py qlib/workflow/recorder.py
• .venv/bin/python -m flake8 --ignore=E501,F541,E266,E402,W503,E731,E203 --per-file-ignores="init.py:F401,F403" qlib