fix(macros): support availability attributes on previews

[Kyle-Ye]

Summary

• Register previews at the #Preview token so availability attributes do not shift their source location.
• Format generated preview closures before registry insertion to avoid extra indentation.
• Add regression tests for attributes before and on the same line as #Preview.

All 16 macro expansion tests pass.

[tuist]

🛠️ Tuist Run Report 🛠️

Tests 🧪

Scheme	Status	Ran test modules	Commit
OpenSwiftUI	✅	7	25f1071bc

Builds 🔨

Scheme	Status	Duration	Commit
OpenSwiftUI	✅	1m 9s	25f1071bc

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.26%. Comparing base (b90679e) to head (9d29694).

Files with missing lines	Patch %	Lines
Sources/OpenSwiftUIMacros/PreviewMacro.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1105   +/-   ##
=======================================
  Coverage   33.26%   33.26%           
=======================================
  Files         805      805           
  Lines       63992    63992           
=======================================
+ Hits        21288    21290    +2     
+ Misses      42704    42702    -2     

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