Don't assume all S_IFREG files are seekable

[rzikm]

Fixes #120577.

It seems that special files like /proc/net/route are not seekable on AzureLinux 3, which trips an assert in SafeFileHandle.Init.
This PR excludes 0-length files (including all pseudofiles) from the assert to unblock CI.

Original outdated description

Since these files report 0 length in fstat, we can relax the seekabilit assumption to only files with Size > 0.

In practice, most file reads are done via RandomAccess.ReadAtOffset and we keep our own seek offset in the SafeFileHandle internally, so seekability does not matter much (the affected unit tests don't fail in release builds), but it breaks in e.g. following scenario where access to SafeFileHandle property would attempt to synchronize the position with the underlying OS handle:

FileStream file = new FileStream("/proc/net/route", FileMode.Open, FileAccess.Read, FileShare.ReadWrite, 10);

System.Console.WriteLine($"Position: {file.Position}, CanSeek: {file.CanSeek}, Length: {file.Length}");

file.Position = 1;

System.Console.WriteLine($"SafeFileHandle: {file.SafeFileHandle}");

This would produce:

Position: 0, CanSeek: True, Length: 0

      System.IO.IOException : Illegal seek : '/proc/net/route'
      Stack Trace:
        /_/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs(104,0): at System.IO.Strategies.OSFileStreamStrategy.get_SafeFileHandle()
...

[Copilot]

Pull Request Overview

This PR fixes an issue where regular files with S_IFREG type were incorrectly assumed to be seekable on all systems. The fix specifically addresses a problem on AzureLinux 3 where special files like /proc/net/route are not seekable despite having the S_IFREG file type, causing assertions to fail in SafeFileHandle.Init.

• Adds a size check to only assume seekability for regular files with size > 0
• Preserves existing behavior for normal files while avoiding assumptions about zero-length special files

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[rzikm]

cc @ManickaP

[adamsitnik]

The change is correct, but I would prefer to just modify the assertion. PTAL at my comment for full picture.

cc @tmds

[ManickaP]

Great catch, thanks!

[rzikm]

Filed #120741 for followup on better CanSeek detection.

[rzikm]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/18584668226

[rzikm]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/18584672863