Make InputRadioGroup preserve child elements

[SteveSandersonMS]

Fixes #31610 and #39145, and supersedes #39208

@MariovanZeist - thanks so much for contributing #39208. I hope you don't mind me coming up with a slightly different solution in this PR. The reasons I wanted to do it differently are:

• The approach in Fix: InputRadioGroup not registering initial click in some circumstances #39208 is to create a custom system of registering for notification to be re-rendered when an ancestor changes. This happens to be equivalent to what CascadingValue does natively when IsFixed is false, so simply by removing the IsFixed=true we get the desired behavior without needing a new mechanism.
• There isn't really any perf drawback to removing the IsFixed=true. Descendants have to be notified to re-render one way or another, because their actual output must change if they become (de)selected. Prior to this fix, the mechanism used @key to recreate the entire child hierarchy every time, whereas now it retains the child hierarchy and just triggers the appropriate re-rendering.

The fix could have been reduced to removing these two lines from InputRadioGroup:

        builder.SetKey(_context);
        builder.AddAttribute(1, "IsFixed", true);

However, in this PR I also chose to reduce allocations by not creating a new InputRadioContext on every render, and instead retaining the existing instance and mutating it in place. It's not particularly complicated to do that but did add quite a few extra changes to the PR diff.

As far as I know, this produces the desired behavior and works as well as the PR at #39208. I verified that it also passes the E2E tests supplied in that PR. But @MariovanZeist, if you have any concerns that the approach here doesn't meet your goals, please let me know!

[SteveSandersonMS]

The nullability changes here and below are just because this code actually does work fine with a null classNames, and the new code benefits by not having to do a ?? string.Empty.

[SteveSandersonMS]

We can also avoid creating a new delegate here on every render.

[MariovanZeist]

@SteveSandersonMS I prefer this solution over mine, as it's cleaner
I didn't know of this particular "side effect" of having IsFixed set to false
I guess due to:

aspnetcore/src/Components/Components/src/CascadingValue.cs

Lines 125 to 128 in 6f5aeb7

	if (_subscribers != null && ChangeDetection.MayHaveChanged(previousValue, Value))
	{
	NotifySubscribers(parameters.Lifetime);
	}

Always good to learn something new.

All concerns addressed in my PR are also addressed here