* refactor: Remove unused `util::is_ready()` function
Commit ffd4c731a2c7c09f34e7b81a16e04bdf91fa973d introduced the
`util::is_ready()` function, which has never been used internally and is
not part of the public API.
This change removes the function.
* refactor: Guard `util::quote_argument()` with `#ifdef __USING_WINDOWS__`
The `util::quote_argument()` function is specific to Windows and is used
in code already guarded by `#ifdef __USING_WINDOWS__`.
* Do not escape double quotes for command line arguments on Windows
This change fixes the handling of double quotes and aligns the behavior
with Python's `Popen` class. For example:
```
>py -3
>>> import subprocess
>>> p = subprocess.Popen("cmd.exe /c dir \"C:\\Program Files\"", stdout=subprocess.PIPE, text=True)
>>> print(f"Captured stdout:\n{stdout}")
```
Currently, the same command line processed by the `quote_argument()`
function looks like `cmd.exe /c dir "\"C:\Program" "Files\""`, which is
broken.
With this change, it looks correct: `cmd.exe /c dir "C:\Program Files"`.
* Add `test_double_quotes` test
The new test ensures that no regressions are introduced in the future.
