fix(files): decrement quota by actual bytes written in stream_write

[joshtrichards]

• Resolves: #tbd/na

Summary

Discovered while investigating random failures in the QuotaTest unit test.

The problem:

• $size is set to the length of $data after truncation.
• The quota ($this->limit) is decremented by $size before the actual write.
• The function returns the result of fwrite(), which may be more or less than $size.

If fwrite() writes fewer bytes than $size (e.g., disk full), the quota will still be reduced by $size, not the actual number written.

Similarly, if it writes more (for example, if the underlying buffer had data in it), the returned value will be higher than expected.

The fix:

• The quota is now decremented by the actual number of bytes written ($written) rather than the intended size.
• This ensures quota tracking stays accurate even if fwrite writes fewer (or more, due to underlying buffering) bytes than requested.

TODO

• It's possible there are other spots with similar behavior; should be checked and followed up on separately as needed
• Backport?

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)