Remove unsafe type narrowing for non-closed TypedDicts

[This issue](https://github.com/microsoft/pyright/issues/10517) attempted to fix a type checking hole related to TypedDict discrimination, but I reverted it because there was no clean way to implement this pattern without PEP 728. Now that PEP 728 is accepted, this unsafe behavior should be reverted.

Start by reading the linked issue #10517 and the details of PEP 728 to understand the earlier TypedDict discrimination fix and why it was reverted. Confirm the intended behavior for non-closed TypedDicts, then verify that unsafe narrowing is no longer accepted with relevant type-checking cases.

Written by the indexing model from the issue text.