Allow users with view permissions to preview unpublished content

[ruibaby]

What type of PR is this?

• Feature
• Bug fix
• Improvement
• Cleanup
• Documentation

What this PR does / why we need it:

Users with post or single-page view permissions currently receive a 404 when previewing unpublished content created by another user. The preview routes only allow contributors and do not consider the user's RBAC permissions.

This change preserves contributor access and also allows users with the corresponding post or single-page view permission to access preview links.

Reproduction:

1. Sign in as a user with the post or single-page view permission who is not a contributor to the content.
2. Open /preview/posts/{name} or /preview/singlepages/{name} for unpublished content created by another user.
3. The preview request returns 404 before this change.

After this change, the same request returns 200 while unauthenticated and anonymous requests remain inaccessible.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

• Added regression coverage for both post and single-page preview routes.
• Verified the focused test suite and Java formatting with Corretto 21.
• This change was implemented with Codex assistance and reviewed through the regression tests below.

Does this PR introduce a user-facing change?

修复拥有文章或页面查看权限的用户无法预览其他用户创建的未发布内容的问题。

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 66.43%. Comparing base (42c374d) to head (427c2cd).
⚠️ Report is 795 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/halo/app/theme/router/PreviewRouterFunction.java	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #10337      +/-   ##
============================================
+ Coverage     59.55%   66.43%   +6.88%     
- Complexity     3812     5264    +1452     
============================================
  Files           677      758      +81     
  Lines         23248    26986    +3738     
  Branches       1500     1904     +404     
============================================
+ Hits          13846    17929    +4083     
+ Misses         8764     8167     -597     
- Partials        638      890     +252     

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