Fix Uri.GetHashCode hash/equality contract violation for file: URIs

[Copilot]

Uri.GetHashCode uses IsUncOrDosPath to select between OrdinalIgnoreCase and ordinal hashing, but Equals uses this.IsUncOrDosPath for comparison mode — so two URIs with identical normalized RemoteUrl but different IsUncOrDosPath flags (e.g. a UNC-derived URI and an explicit file:///) compare equal yet produce different hash codes.

var uri1 = new Uri("\\\\\u202a"); // UNC path, IsUncOrDosPath=true → normalizes to file:///
var uri2 = new Uri("file:///");   // IsUncOrDosPath=false

Console.WriteLine(uri1 == uri2);        // True
Console.WriteLine(uri1.GetHashCode());  // -1626931478  ← mismatch
Console.WriteLine(uri2.GetHashCode());  //  558067402   ← mismatch

Description

Root cause: Flags.DosPath and Flags.UncPath are only ever set alongside syntax = UriParser.FileUri, so IsUncOrDosPath ⇒ IsFile is a provable invariant. A plain file:/// URI never sets those flags, so GetHashCode used a different hash algorithm than a UNC-derived URI with the same normalized form.

Fix: Replace if (IsUncOrDosPath) with if (IsFile) in GetHashCode. This extends OrdinalIgnoreCase hashing to all file: URIs, ensuring consistent hash codes for any two file: URIs that Equals can report as equal. No existing behavior changes for UNC/DOS paths; the only difference is that plain file: URIs now also use OrdinalIgnoreCase.GetHashCode.

Test: Added new Uri("\\\\\u202a") vs new Uri("file:///") to Equals_TestData, which is already consumed by EqualsTest — a harness that automatically verifies both equality symmetry and hash code consistency for equal pairs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Uri GetHashCode mismatch for instances that compare as Equal</issue_title>
<issue_description>```c#
var uri1 = new Uri("\\\u202a");
var uri2 = new Uri("file:///");

Console.WriteLine(uri1 == uri2);
Console.WriteLine(uri1.GetHashCode());
Console.WriteLine(uri2.GetHashCode());

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Uri GetHashCode mismatch for instances that compare as Equal #124651

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