fix: accept a FIFO .env file in dotenv run

[sungbin1015]

Problem

dotenv run rejects a .env path that is a FIFO, although the library (load_dotenv, dotenv_values, find_dotenv, since #586) and the list/get subcommands read it fine:

$ dotenv -f <(echo A=1) list
A=1
$ dotenv -f <(echo A=1) get A
1
$ dotenv -f <(echo A=1) run printenv A
Usage: dotenv run [OPTIONS] [COMMANDLINE]...
Try 'dotenv run --help' for help.

Error: Invalid value: Invalid value for '-f' "/dev/fd/63" does not exist.

The same happens with a named pipe created by mkfifo and picked up as the default ./.env, which is the 1Password-mounted .env case #586 was written for: load_dotenv() works there, dotenv run -- cmd does not.

Cause

run guards the path with os.path.isfile(file) before handing it to DotEnv. DotEnv._get_stream and find_dotenv use _is_file_or_fifo instead; the guard in cli.py was not updated when FIFO support was added.

Fix

Use _is_file_or_fifo in the run guard. Missing paths and directories are still rejected with the same message. No change on Windows, where _is_file_or_fifo is equivalent to os.path.isfile.

Tests

• tests/test_fifo_dotenv.py::test_cli_run_from_fifo (Unix only, like the existing FIFO test): fails before the change with the error above, passes after.
• pytest: 285 passed, 4 skipped. ruff check src tests, ruff format --check src tests and mypy for 3.10-3.14 are clean.

Related work

• Support reading .env from FIFOs (Unix) #586 (merged) added FIFO support to the library functions; this PR extends the same check to the CLI's run path, which Support reading .env from FIFOs (Unix) #586 did not cover.
• feat: Add waiting logic for FIFO files that may not be immediately readable #599 (open, for Bug: python-dotenv doesn't load 1Password “mounted” .env files (looks like a FIFO / fd-backed file) #598) adds waiting logic in src/dotenv/main.py for FIFOs that are not immediately readable. It does not touch cli.py, so dotenv run would still reject a FIFO with it applied.
• Resolve the default CLI env path on each invocation #718 (open) also edits cli.py, but for a different problem (when the default .env path is resolved).