Unable to truncate file when using local storage

Description

When trying to truncate a file while using local storage, an error is raised even if the provided mode is correct. This may be due to the missing implementation of truncate in LocalFileOpener.

To Reproduce

The following code raises io.UnsupportedOperation: truncate:

import fsspec

with fsspec.open("my_file.txt", mode="wb+") as stream:
    stream.write(b"hello")
    stream.truncate(2)

While this one works:

import fsspec

with fsspec.open("my_file.txt", mode="wb+") as stream:
    stream.write(b"hello")
    stream.f.truncate(2)

Environment

Python : 3.11.2
fsspec : 2023.6.0

Start by running the provided Python reproduction, then inspect LocalFileOpener in fsspec/implementations/local.py around the referenced line. Confirm how stream.truncate differs from stream.f.truncate and check the surrounding local-file behavior. Done means the public stream.truncate(2) call works for the shown wb+ example without raising io.UnsupportedOperation.