fix: Files[io].writeAll does not create file unnecessarily. #3367

[ankitkumarrain]

Summary

Fixes #3667

Bug

writeAll used Stream.resource(writeCursor(...)) which eagerly opens
and creates the file during resource acquisition — before any bytes
are pulled from the input stream.

Fix

Use pull.stepLeg to peek at the input stream first:
Stepleg takes first chunk scpoe safely.

• Empty stream → file is never created
• Erroring stream → error propagates, file never created
• Normal stream → behavior unchanged

Files Changed

• io/shared/src/main/scala/fs2/io/file/Files.scala
• io/js/src/main/scala/fs2/io/file/FilesPlatform.scala

[ankitkumarrain]

@mpilquist , Can you please review this code .

[armanbilge]

@ankitkumarrain thanks for the PR. Before we review it would you mind making a contribution to our onboarding repository? Directions are in the README.

https://github.com/typelevel/gsoc-onboarding

Please review our AI policy and update your PR accordingly.

https://typelevel.org/gsoc/ai.html

[ankitkumarrain]

@armanbilge Thanks for comment on this pull request .I have read AI policy and updated pull request Accordingly. Can you please review this pull request as i understand the code i have written. I visited to onboarding repository that was very fantastic i would really like to add my component there . Thank you for your time.
Best Regards !
Ankit
@ankitkumarrain

[mpilquist]

Hi Ankit -- please be sure to complete the onboarding task first.

Re: this PR, what's the behavior when writing an empty stream to a file? What's the desired behavior?