--variables-file fails on FIFO/named pipes (partial read)

What is the current bug behavior?

--variables-file fails when the file is a FIFO (named pipe). Hurl does a single read() on the file descriptor and gets whatever bytes are in the pipe buffer at that instant — often a partial chunk — instead of looping until EOF. The variables are left empty, truncated, or parsed from a fragment of a comment line, causing Undefined variable / Missing value for variable / Assert status code errors.

Steps to reproduce

1. Create a FIFO and a test hurl file:

mkdir /tmp/hurl-fifo-repro && cd /tmp/hurl-fifo-repro
mkfifo vars.env
printf 'GET https://httpbin.org/get\nX-Api-Key: {{api_key}}\nHTTP 200\n' > test.hurl

1. Control — regular file (works):

printf 'api_key=fakevalue123\n' > vars-regular.env
hurl --variables-file vars-regular.env test.hurl
# exit 0, HTTP 200

1. FIFO — fails 5/5:

for i in 1 2 3 4 5; do
  (printf 'api_key=fakevalue123\n' > vars.env) &
  hurl --variables-file vars.env test.hurl 2>&1 | head -1
  echo "attempt $i exit: ${PIPESTATUS[0]}"
done
# error: Assert status code  (or "Undefined variable" / "Missing value for variable"
#                                depending on which partial chunk arrives)
# exit 4 (or 3), 5/5 failures

The error varies depending on which bytes arrive in the partial read:

• Empty chunk → Undefined variable (exit 3)
• Comment fragment parsed as name=value → Missing value for variable <fragment> (exit 1)
• Truncated key=value → empty/partial value sent to the request → Assert status code (exit 4)

What is the expected correct behavior?

--variables-file should loop on read() until EOF (0 bytes returned), accumulating the full file contents before parsing — the same way cat, grep, and other standard tools handle FIFOs. A FIFO is a valid file type per POSIX, and open() + read() work on it; the reader just needs to drain the pipe completely.

Execution context

• OS: macOS 15 (Darwin 25.5.0)
• Hurl Version: 8.0.1
• Found via 1Password Environments (beta), which mounts .env files as FIFOs fed on demand by the 1Password desktop app instead of writing plaintext secrets to disk. --variables-file fails ~100% of the time on these mounts — the real-world trigger for this report.

Possible fixes

The variables-file reader likely uses a single read() or read_to_end() equivalent that doesn't loop on pipes. Replacing it with a loop-until-EOF read (e.g., std::io::Read::read_to_end / read_to_string which loop internally, or an explicit loop) should fix it. Rust's std::fs::File → std::io::Read::read_to_string handles this correctly because it loops until EOF.