Add InactiveClass parameter to NavLink

[KitKeen]

Add InactiveClass parameter to NavLink
Fixes #37765

Summary

Adds an optional InactiveClass parameter to <NavLink> that mirrors ActiveClass. When the link's href does not match the current URI, the supplied class is appended to any user-provided class. The default is null, so existing usage is unaffected.

Example

<NavLink class="bg-dark-gray" ActiveClass="text-white" InactiveClass="text-gray" href="/page" />
- Active: <a class="bg-dark-gray text-white">
- Inactive: <a class="bg-dark-gray text-gray">

Implementation

Refactored UpdateCssClass() to pick ActiveClass (or "active" default) when active, or InactiveClass when inactive. Same CombineWithSpace helper.

Tests

Added 5 cases to NavLinkTest.cs:
- Active state ignores InactiveClass
- Inactive state applies InactiveClass
- Inactive without InactiveClass preserves prior behavior
- Inactive combines user class with InactiveClass
- Active combines user class with ActiveClass even when InactiveClass set

Public API

PublicAPI.Unshipped.txt updated with the new property.

Note

This is a public API addition. The shape was proposed in #37765 and matches the existing ActiveClass pattern, so should be uncontroversial — but flagging in case the team wants formal API review.

[dotnet-policy-service]

Thanks for your PR, @KitKeen. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.