Refactor browser tool PATH construction to include Termux directories
(/data/data/com.termux/files/usr/bin, /data/data/com.termux/files/usr/sbin)
so agent-browser and npx are discoverable on Android/Termux.
Extracts _browser_candidate_path_dirs() and _merge_browser_path() helpers
to centralize PATH construction shared between _find_agent_browser() and
_run_browser_command(), replacing duplicated inline logic.
Also fixes os.pathsep usage (was hardcoded ':') for cross-platform correctness.
Cherry-picked from PR #9846.