The size probe every read path starts with — `wc -c < path` — opens the
path. On a FIFO with no writer, a socket, or a character device that never
reaches EOF, that read never returns, and read_file/read_file_raw/
read_file_bytes all pass no timeout to _exec. The turn wedges until the
process is killed.
The device blocklist in tools/file_tools.py cannot close this: it matches
literal /dev/* names, so it can only ever cover paths someone thought to
enumerate. A FIFO is a file type and can sit at any path.
Gate the probe behind `[ -f ]`, which stats instead of opening, and report
a path that exists but is not a regular file as such. A missing path keeps
its existing not-found handling.
