Write the received file from the stream for handle based FileStreams

[SimonCropp]

FileComparer.InnerCompare writes the received file on a mismatch with IoHelpers.CopyFile(fileStream.Name, …). Name is not a path for a FileStream built from a handle — .NET Framework reports [Unknown], and modern .NET falls back to that when the path cannot be resolved from the handle.

The first run of such a test passes: the New path goes through IoHelpers.WriteStream, which already guards for this (TryFileCopy bails on a non rooted name and copies through the handle instead). The next run, once a .verified. file exists and the content differs, throws:

System.Exception : Failed to compare files:
---- System.IO.FileNotFoundException : Could not find file '[Unknown]'.

So WriteStream replaces the raw copy here — same copy-by-path fast path, with the fallback the other paths already get.

HandleStreamTests.Mismatch opens a FileStream over another stream's SafeFileHandle and verifies it against a differing snapshot. Two notes on how it is built:

• It uses a binary extension. With a text extension the stream is read into a StringBuilder up front and FileComparer is never reached, so the test would pass without proving anything.
• The two contents are the same length, so the length fast path (which calls WriteStream and is therefore already safe) does not short-circuit ahead of the comparison.

On net48 it reproduces the crash above; on net11.0 the handle resolves back to a real path, so it passes either way there and only asserts the outcome. Full Verify.Tests passes on both net48 (1221) and net11.0 (1300).