Fix: Should not throw on ARJ detection

[adamhathcock]

Same as #1066

Fixes #1064

[Copilot]

Pull request overview

This PR fixes issue #1064 where ArjFactory.IsArchive would throw exceptions when attempting to detect non-ARJ files, rather than returning false as expected. The fix wraps the ARJ header detection logic in a try-catch block to handle invalid data gracefully.

• Adds exception handling to ArjFactory.IsArchive to catch exceptions during header parsing
• Adds two test cases to verify the fix: one for non-archive files and one for valid archives

Reviewed changes

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

File	Description
src/SharpCompress/Factories/ArjFactory.cs	Adds try-catch block around ARJ header reading to return false instead of throwing on invalid data
tests/SharpCompress.Test/ArchiveTests.cs	Adds regression tests for IsArchive method behavior with both invalid and valid archive files

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