Fix edge-case Uri IPv4 parsing regression

[MihaZupan]

Recently broken in #121225

If the Uri ends with the host (no path/query/fragment delimiter) and has trailing whitespace, we'd pass the host including the whitespace to ParseNonCanonical which would return Invalid, hitting the Debug.Assert(result != Invalid) below.

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[Copilot]

Pull request overview

This PR fixes a regression in URI IPv4 address parsing when the URI ends with the host and has trailing whitespace. The bug was introduced in PR #121225 and would cause a Debug.Assert failure when ParseNonCanonical returned Invalid after receiving a host string that included trailing whitespace.

Key Changes:

• Fixed IPv4AddressHelper.ParseCanonicalName to pass the correct span length when calling ParseNonCanonical
• Added a regression test to verify URIs with trailing whitespace are parsed correctly

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.Uri/src/System/IPv4AddressHelper.cs	Fixed the span slice to use explicit length (end - start) instead of implicitly including all characters from start to end of string, preventing trailing whitespace from being passed to ParseNonCanonical
src/libraries/System.Private.Uri/tests/FunctionalTests/UncTest.cs	Added regression test for UNC URIs with IPv4 hosts and trailing whitespace (newline character)

[EgorBo]

Thanks for noticing!