Remove unsafe code from Uri PrivateParseMinimal, CheckAuthorityHelper, and related helpers

[MihaZupan]

Removes unsafe code from PrivateParseMinimal, EscapeUnescapeIri, UncNameHelper.IsValid, GetHostViaCustomSyntax, CheckAuthorityHelper.

[Copilot]

Pull Request Overview

This PR removes unsafe code from URI parsing methods by converting pointer-based implementations to use ReadOnlySpan<char>. The changes eliminate the use of fixed statements and pointer arithmetic while preserving the original parsing logic.

Key Changes:

• Converted IriHelper.EscapeUnescapeIri from unsafe pointer-based to safe span-based implementation
• Converted UncNameHelper.IsValid to accept ReadOnlySpan<char> instead of char* pointer
• Converted Uri.PrivateParseMinimal, CheckAuthorityHelper, and GetHostViaCustomSyntax to use spans instead of pointers

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Private.Uri/tests/UnitTests/IriEscapeUnescapeTest.cs	Simplified tests by removing unsafe code and HeapCheck buffer validation class, now calling IriHelper.EscapeUnescapeIri directly
src/libraries/System.Private.Uri/src/System/UriExt.cs	Updated EscapeUnescapeIri wrapper to use span-based API instead of unsafe pointer-based call
src/libraries/System.Private.Uri/src/System/IriHelper.cs	Converted EscapeUnescapeIri from unsafe pointer arithmetic to safe span indexing while preserving all parsing logic
src/libraries/System.Private.Uri/src/System/UncNameHelper.cs	Converted IsValid from unsafe pointer-based to span-based, using uint cast for bounds checking and returning output parameter for name length
src/libraries/System.Private.Uri/src/System/Uri.cs	Removed unsafe code from PrivateParseMinimal, CheckAuthorityHelper, and GetHostViaCustomSyntax by converting to span-based parsing while maintaining all parsing behavior and edge cases