Simplify InitializeUri and remove some unreachable branches

[MihaZupan]

The current initialization logic is written in a way that makes it hard to read, and hides inconsistencise / unreachable logic.

This PR only shuffles things around & removes some unreachable branches without changing the behavior.
The aim is to make future changes that do change behavior actually reviewable.

Some context:

• we use _syntax to represent the Uri scheme. It's always null for relative Uris and set for absolute ones
• before we call into the InitializeUri we try to parse the scheme. This populates both ParsingError err and _syntax. These two are mutually exclusive - if err is set it means that _syntax == null and vice-versa.

I've split this up into smaller commits:

• a6523bb?w=1
  • This recognizes the assertion I made above about the error and syntax being mutually exclusive.
  • It moves the logic for err != ParsingError.None up to the top, allowing us to drop the extra nesting of the if (_syntax != null) branch below it (it'll always be true).
  • It also removes two dead branches that were checking if err != ParsingError.None when we know it can't be
• c73795e
  • This simplifies our logic that decides whether an implicit file should be trated as absolute or relative.
  • We were effectively checking NotAny(Flags.DosPath) && uriKind == UriKind.Relative in one place and InFact(Flags.DosPath) && uriKind == UriKind.Relative in another, while doing the same thing in both cases. I simplified that condition to just check for UriKind.Relative.
  • After you handle the Relative case, the uriKind != UriKind.Absolute check becomes == UriKind.RelativeOrAbsolute, which makes it clearer what we're doing
• fdcd584
  • Changes the UriFormatException to the return value instead of an out param
• 7400e9a
  • Takes a block of code that's duplicated at the end of both the if and else and moves it after the branch
• 393f78f
  • Inverts one conditon to drop some nesting
• 2f96867
  • Simple change of the switch block to the switch expression + removes an unused enum value

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[Copilot]

Pull request overview

This PR refactors the InitializeUri method to improve readability and remove unreachable code paths without changing behavior. The changes reorganize the initialization logic to make it clearer which conditions are mutually exclusive and to eliminate dead branches.

Key changes:

• Reorganized InitializeUri to handle errors first, then process valid absolute URIs
• Changed InitializeUri to return UriFormatException? instead of using an out parameter
• Converted GetException method to use a switch expression with UnreachableException for the default case

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
UriExt.cs	Refactored InitializeUri method to simplify control flow, removed unreachable branches, and changed exception handling from out parameter to return value
UriEnumTypes.cs	Removed unused NonEmptyHost enum value from ParsingError
Uri.cs	Converted GetException from a switch statement to a switch expression and changed return type from nullable to non-nullable