fix: return ErrUnexpectedEOF on short copies

[hayabusa-cloud]

Summary

• CopyNPolicy and CopyNBufferPolicy returned (written, nil) on short copies instead of (written, io.ErrUnexpectedEOF), diverging from the CopyN/CopyNBuffer contract.
  • Solution: Add the same written < n post-check after copyBufferPolicy returns.
• Fix copyBufferPolicy slow-path write loop to check IsSemantic(ew) with rollbackSeeker before the bare return.
• Update two tests to assert the corrected io.ErrUnexpectedEOF behavior.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
io.go	86.66%	1 Missing and 1 partial ⚠️

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[Copilot]

Pull request overview

This pull request fixes a critical bug where CopyNPolicy and CopyNBufferPolicy violated the CopyN contract by returning (written, nil) instead of (written, io.ErrUnexpectedEOF) when unable to copy the requested n bytes. The fix also adds defensive handling for wrapped semantic errors in the write loop.

Changes:

• Added post-copy validation in CopyNPolicy and CopyNBufferPolicy to return io.ErrUnexpectedEOF when written < n and the error is nil or EOF
• Added rollback handling for wrapped semantic errors in copyBufferPolicy write loop
• Updated test expectations to assert the corrected io.ErrUnexpectedEOF behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
io.go	Fixed CopyNPolicy and CopyNBufferPolicy to return io.ErrUnexpectedEOF on short copies; added defensive rollback for wrapped semantic errors in copyBufferPolicy
policy_test.go	Updated test names and assertions to expect io.ErrUnexpectedEOF on short copies