Reset all Uri fields when reusing an instance

[MihaZupan]

Simplifying OriginalString to _originalUnicodeString ?? _string in #33044 introduced a bug affecting the new Uri(baseUri, relativeUri) combine operations.

The bug can be hit if:

1. The relativeUri contains non-ascii characters
2. The baseUri uses a custom (not built-in scheme) - in this case pack
3. A custom UriParser is registered for that scheme
4. That parser is a "Simple parser" - a GenericUriParser, and it is set up without the GenericUriParserOptions.IriParsing flag

The cause is that when combining the Uris, the Uri instance gets reused, but not all internal fields are reset. As _originalUnicodeString was not being reset, it changed the output of OriginalString when reused. This led to the relative Uri being used in the parsing logic instead of the constructed combined Uri.

This does not affect built-in schemes / custom parsers with the IriParsing flag, as _originalUnicodeString will be properly set during parsing (the old value won't sneak through).

Fixes #41177

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[MihaZupan]

cc: @ryalanms

[ryalanms]

Thank you, @MihaZupan and @karelz, for the fast response and fix!

[ryalanms]

cc @dotnet/wpf-developers

[ManickaP]

LGTM

[MihaZupan]

/backport to release/5.0

[github-actions]

Started backporting to release/5.0: https://github.com/dotnet/runtime/actions/runs/225384771