Preserve TextInput returnKeyType when changing multiline

[jamfromouterspace]

Summary:

RCTCopyBackedTextInput is called when a TextInput switches between
single-line and multiline and the Fabric component view swaps its backing
UIKit view (RCTUITextField <-> RCTUITextView). It copies the keyboard
traits across to the replacement view, but returnKeyType is not among them —
keyboardType is copied on the line directly above it.

Because the prop itself has not changed across the transition, the prop diff in
RCTTextInputComponentView sees done -> done and does not re-run the
setter. The replacement view therefore keeps its default Return key, so an
input declared returnKeyType="done" renders the newline key instead of Done
once it has flipped to multiline.

This is the same class of omission that was fixed for tintColor in #57748,
and the fix is the same shape: copy the property along with its neighbours.

Repro: a TextInput with a stable key and returnKeyType="done" whose
multiline prop goes false -> true. After the transition, focusing it shows
the default Return key. Fabric view recycling can produce the same backing-view
transition even for an input that is always multiline, which makes it
intermittent in real apps.

Verified against 0.86.3; the line is still missing on 0.87.1 and on main.

Changelog:

[IOS] [FIXED] - Preserve TextInput returnKeyType when switching between single-line and multiline backing views.

Test Plan:

Added testCopyBackedTextInputPreservesReturnKeyType to
React/Tests/Text/RCTTextInputUtilsTest.mm, alongside the existing
testCopyBackedTextInputPreservesTintColor added in #57748. It sets
returnKeyType = UIReturnKeyDone on an RCTUITextField, runs
RCTCopyBackedTextInput into an RCTUITextView, and asserts the value
survives. The test fails without the one-line change and passes with it.

Also confirmed in a production app: a multiline TextInput with
returnKeyType="done" intermittently displayed the newline key, and adding
this single line fixed it.

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