Fix rendering of foreignObject in SVGs

[captainsafia]

Resolves #30973.

SVG supports a foreignObject element that can be used to display arbitrary HTML within an SVG. Particularly for Blazor, this HTML can represent a render fragment or Blazor component, like so:

<svg id="svg-with-two-way-binding">
    <foreignObject width="200" height="200">
        <div>
            <input @bind="value" @bind:event="oninput" type="number"/> Two-way binding
        </div>
    </foreignObject>
</svg>

Currently, our client-side rendering logic renders the SVG element and all its children under the SVG namespace by invoking document.createElementNS('http://www.w3.org/2000/svg', tagName). However, for the browser to correctly render the HTML content under the foreignObject it must be rendered under an XHTML namespace.

To resolve this, we add an additional check to set the namespace to XHTML for elements that are children of a foreignObject.

Previous:

- svg (SVG namespace)
  - foreignObject (SVG namespace)
    - div (SVG namespace)
      - input (SVG namespace)

Current:

- svg (SVG namespace)
  - foreignObject (SVG namespace)
    - div (XHTML namespace)
      - input (XHTML namespace)

[javiercn]

Looks great!

[SteveSandersonMS]

Note that this isn't the only place where we check isSvgElement(parent). We also do that in insertMarkup to vary how we parse markup strings. My guess is that we'd also need to update insertMarkup in the same way as this in case we're inserting a markup block as the top-level thing inside a foreign element. Does that sound correct to you?

If this is all true, it suggests to me that instead of having both isSvgElement and isForeignObject as separate things we have to remember to use together, we should instead replace them both with a single shouldInsertImmediateChildrenAsSvg function. This would also have an efficiency benefit, since that function would only need to evaluate getClosestDomElement(element) once instead of two methods both evaluating that.

[captainsafia]

Great catch!

Yep -- it makes sense to refactor things a bit with an efficiency focus if these check happens more than once.

Will update PR accordingly.