fix(utils): report buffered file write failures

[unbridled-41]

Summary

Return an error when a buffered file write fails during stream flush or close.

Root cause

writeFile() checked out.bad() immediately after inserting data into the stream. A buffered stream can accept the insertion and only surface an error while flushing or closing, so callers could receive success even though the file contents were not written.

A reproducible example is /dev/full: it opens successfully but reports ENOSPC when buffered data is flushed.

Changes

• Close the output stream before reporting success.
• Check the stream failure state after close.
• Add a regression test using /dev/full, skipped on platforms where it is unavailable.

Reproduction

Call linglong::utils::writeFile("/dev/full", "test data") on Linux. Before this change it could report success; after this change it returns an error.

Test plan

• Release build
• ll-tests --gtest_filter='FileTest.WriteFileToDevFull'
• git diff --check

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: unbridled-41

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[deepin-ci-robot]

Hi @unbridled-41. Thanks for your PR.

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