fix: handle AlreadyExists race in create_dir_all

[raema]

Problem

monoio::fs::create_dir_all can return AlreadyExists when two tasks create the same directory at the same time.

Approach

If mkdir fails, check whether the path is now a directory. If it is, treat the operation as successful. Otherwise return the original error.

The fix adds a check after mkdir:

• mkdir succeeds -> OK
• mkdir fails but is_dir() is true -> OK (another task already created it)
• otherwise -> return the error

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