Fix null IVolume.FileName for single-volume file-based archives

[Copilot]

archive.Volumes could return null for FileName when opening a single archive file (for example ArchiveFactory.Open("example.zip")). The volume stream in this path is often SourceStream-backed rather than FileStream-backed, so filename metadata was not surfaced.

• Volume filename resolution

  • Updated Volume.FileName to preserve existing FileStream behavior and add a fallback for SourceStream by reading the first file part path.
  • This keeps multi-part behavior intact while fixing single-file archive open paths.

• Regression coverage

  • Added ArchiveFactoryTests.OpenArchive_SingleVolume_VolumeFileName_MatchesPath.
  • Verifies Assert.Single(archive.Volumes).FileName matches the input file path for representative single-volume archives (zip, tar, rar, 7z).

public string? FileName =>
    (_baseStream as FileStream)?.Name
    ?? (_baseStream as SourceStream)?.Files.FirstOrDefault()?.FullName;

[kilo-code-bot]

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[Copilot]

Pull request overview

Fixes IVolume.FileName being null when opening single-volume archives from a file path (where the underlying volume stream is often a SourceStream rather than a FileStream), and adds a regression test to ensure the archive path is surfaced via archive.Volumes.Single().FileName.

Changes:

• Updated Volume.FileName to fall back to the first SourceStream file part’s FileInfo.FullName when the underlying stream isn’t a FileStream.
• Added a parameterized test covering representative single-volume formats (zip/tar/rar/7z) to ensure Volume.FileName matches the opened archive path.

Reviewed changes

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

File	Description
src/SharpCompress/Common/Volume.cs	Adds SourceStream-based filename fallback for single-volume, file-path archive opens.
tests/SharpCompress.Test/ArchiveFactoryTests.cs	Adds regression test asserting archive.Volumes.Single().FileName matches the input archive path.

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