Fix: Subtree hints not propagating correctly (#94489)
Fixes a mistake in the client when constructing a route tree from a
partial server response. When applying the patch, we must propagate the
new subtree hints all the way up to the root.
This was an existing bug that I found when implementing the next PR in
the stack, which relies on the subtree hint mechanism. I've split it out
into its own step and confirmed the bug + fix using a regression test
expressed via existing behavior.
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