chore(ux): improve oras backup error message

[Wwwsylvia]

What this PR does / why we need it:

This PR improves the error message in oras backup for the case where the output path ends with .tar but is an existing directory.

BEFORE:

Error: unable to create output file test.tar: open test.tar: is a directory

AFTER:

Error: the output path test.tar already exists and is a directory: open test.tar: is a directory
To back up to a tar archive, please specify a different output file name or remove the existing directory.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1831

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.25%. Comparing base (97cb6f7) to head (29b3c8c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1833      +/-   ##
==========================================
+ Coverage   85.21%   85.25%   +0.04%     
==========================================
  Files         143      143              
  Lines        6737     6742       +5     
==========================================
+ Hits         5741     5748       +7     
+ Misses        708      707       -1     
+ Partials      288      287       -1     

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[Copilot]

Pull Request Overview

This PR improves the error message when using oras backup with an output path that ends with .tar but points to an existing directory instead of a file. The enhancement provides clearer guidance to users about the conflict and suggests actionable solutions.

• Enhanced error handling to detect directory conflicts when creating tar archives
• Added user-friendly error message with specific recommendations
• Included comprehensive test coverage for the new error scenario

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cmd/oras/root/backup.go	Added directory detection logic and improved error message for tar output conflicts
test/e2e/suite/command/backup.go	Added end-to-end test to verify the new error message behavior

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[shizhMSFT]

LGTM