fix(react): guard against undefined $$typeof in check()

[Clickin]

Changes

Adds a null guard in the check() function in @astrojs/react's server renderer to prevent TypeError when Component["$$typeof"] is undefined on non-React component objects.

Background

When @astrojs/react is used alongside other framework integrations (e.g. @astrojs/svelte), the check() function may receive non-React component objects. The current code calls .toString() on Component["$$typeof"] without checking if it exists:

if (typeof Component === "object") {
    return Component["$$typeof"].toString()... // TypeError when $$typeof is undefined
}

Fix

Extract $$typeof into a local variable and use != null short-circuit, returning false for components that lack the property:

if (typeof Component === "object") {
    const $$typeof = Component["$$typeof"];
    return $$typeof != null && $$typeof.toString().slice("Symbol(".length).startsWith("react");
}

Related issue

Closes #17552

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