This PR fixes #93961.

Explanation

The URI class holds different information in the path property when the file was opened through a UNC location. For this fix I used the property fsPath instead of path in the onDidClickPreviewLink handler.

For a local file the URI class holds:

where as for a UNC opened file the URI class holds:

Here is an excerpt from uri.ts:

      foo://example.com:8042/over/there?name=ferret#nose
      \_/   \______________/\_________/ \_________/ \__/
       |           |            |            |        |
    scheme     authority       path        query   fragment
       |   _____________________|__
      / \ /                        \
      urn:example:animal:ferret:nose

Test the issue and the fix:

1. To test this issue create two md files (see below this list).

2. Open the readme.md file with an UNC path like "\\server\sharedfolder\readme.md".

3. Open the preview of the markdown file readme.md.

4. In the preview window, click on the Relative link to TEST.md link.

5. The test.md file should open in the preview. Without the fix, an error message is being shown.

Test files

readme.md:
Here's a link: [Relative link to TEST.md](TEST.md)

test.md:
This is the test markdown file