fix(runtime-core): Ensure raw Slots are only normalized once by withContext (Fix: #5353)

[LinusBorg]

This PR ensures that an already normalized slot isn't normalized again, which could potentially lead to a "max stack" error from too many nested function calls (aside from simply being unnecessary work).

I didn't add a test case as this change doesn't have a public effect I could test against - if all tests still pass this should be fine.

But I can come up with one if necessary.

close: #5353

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