Slai Agent Runtime：Use POSIX paths for Linux mounted patches

[Slai.life (huong8373tt-beep)]

Slai Agent Runtime contribution

This pull request is submitted by Slai Agent Runtime via the Slai.life GitHub account, @huong8373tt-beep.

• build the path to a patch mounted in a Linux task container with posixpath.join, independently of the host OS
• add a focused regression test that exercises LinuxRuntime.apply_patch and verifies that its git apply and cleanup commands use /mnt_tmp/<file> without backslashes

Why

When RepoLaunch runs on a Windows host but launches a Linux task container, os.path.join("/mnt_tmp", filename) produces a backslash. Bash consumes that backslash as an escape, so git apply cannot open the mounted patch and evaluation stops before the candidate can be applied or tested.

Validation

• D:\swebench\live_cpp142_linux\.venv_eval_py312\Scripts\python.exe -m pytest tests/runtime_test.py -q -p no:cacheprovider --basetemp=D:\swebench\live_cpp142_linux\runtime\pytest_repolaunch_pr59 -k linux_runtime_uses_posix_path_for_mounted_patch — 1 passed, 6 deselected
• D:\swebench\live_cpp142_linux\.venv_eval_py312\Scripts\python.exe -m py_compile launch/core/platforms/linux.py tests/runtime_test.py
• git diff --check

This is intentionally separate from PR #57, which normalizes patch bytes (UTF-8, no BOM, LF-only). This PR fixes Linux container path construction.