Fix duplicate Uri ToString for DOS-like custom schemes

[MihaZupan]

The fix in #36429 wasn't quite right because DOS-like Uris may appear with non-file schemes (e.g. custom:C:/uri).
This changes the condition to handle such inputs (Dos || Unix instead of File && !Unc).

The change with checking for InFact(Flags.AuthorityFound) is specific for custom schemes with DOS-like inputs.
It ensures that custom:C:/uri and custom:C:/üri (with non-ASCII or not) behave similarly with ToString and preserve whether : vs :// was used.

Fixes #94572

[Copilot]

Pull request overview

This PR fixes a bug in URI ToString() handling for DOS-like paths in custom (non-file) schemes. The issue was that the previous fix only handled file:// URIs, but DOS-like paths can appear with any custom scheme (e.g., custom:C:/uri).

Key Changes:

• Modified the condition in ParseRemaining to check for DOS/Unix paths regardless of scheme (not just file://)
• Updated CreateUriInfo to handle cases where AuthorityFound is set but there are no double slashes (DOS path immediately after scheme)
• Added logic to preserve whether : vs :// was used in the original URI for custom schemes with DOS paths

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.Uri/src/System/Uri.cs	Fixed URI string reconstruction logic to handle DOS paths in custom schemes; added conditional handling for authority delimiters based on whether the original URI used : or ://
src/libraries/System.Private.Uri/tests/FunctionalTests/UriTests.cs	Added comprehensive test cases for DOS paths in file:// scheme and custom schemes, including vsmacros scheme behavior