feat: Add waiting logic for FIFO files that may not be immediately readable

[lukeocodes]

Fixes #598

python-dotenv already had basic FIFO support (added in #586), it didn't work with asynchronously mounted FIFO files, like by 1Password's new (beta) Environment feat

Existing implementation opened FIFOs directly:

with open(self.dotenv_path, encoding=self.encoding) as stream:
    yield stream

The flaw with this approach: when a FIFO exists but isn't immediately ready to read (common with async-mounted filesystems), open() can block indefinitely waiting for a writer, or read it as empty.

The existing test (test_load_dotenv_from_fifo) worked because it started a writer thread before attempting to read, ensuring the FIFO was ready and had content

Solution

I've added a new _wait_for_file_content() function that handles FIFOs that may not be immediately readable:

1. Detects FIFOs using stat.S_ISFIFO() to distinguish them from regular files
2. Uses select.select() to wait for the FIFO to become readable (with a 5-second timeout)
3. Opens in binary mode with unbuffered I/O (rb, buffering=0) for accurate readiness detection
4. Reads content once when ready and returns it as a StringIO object
5. Handles regular files normally without any waiting overhead

The result is that even when using regular FIFO or using 1Password, it reads (or prompts 1Password for an unlock password) correctly

Changes

• Added _wait_for_file_content() function in src/dotenv/main.py
• Updated _get_stream() to use the new function for FIFO-aware file reading

Testing

• All existing tests pass (210/210)
• New tests verify FIFOs are properly handled with waiting logic

Impact

This change is backward compatible and imprpves support for:

• 1Password mounted files
• Other mounted filesystems
• Any FIFO that may not be immediately readable when first accessed (5 sec timeout)