Handle special negative values in codePoints

[Noneangel]

Is your feature request related to a problem? Please describe.
I would like to chain negative code points using multi_text_key.

Describe the solution you'd like
In the wiki, it is stated that code support special negative values (listed in checkAndConvertCode).
Could it be possible to use those negative values in codePoints ?
Currently, this layout.json containing { "$": "multi_text_key", "codePoints": [-34, -11], "label": "U" } can not be imported: I have this error from the logs.

2025-02-19 23:51:43.740673 W CustomLayoutUtils: json layout parsed, but considered invalid
java.lang.IllegalArgumentException: Not a valid Unicode code point: 0xFFFFFFDE
	at java.lang.Character.toChars(Character.java:9241)
	at java.lang.StringFactory.newStringFromCodePoints(StringFactory.java:161)
	at helium314.keyboard.keyboard.internal.keyboard_parser.floris.KeyData$DefaultImpls.toKeyParams(TextKeyData.kt:402)
	at helium314.keyboard.keyboard.internal.keyboard_parser.floris.MultiTextKeyData.toKeyParams(TextKeyData.kt:659)
	at helium314.keyboard.keyboard.internal.keyboard_parser.floris.KeyData$DefaultImpls.toKeyParams$default(TextKeyData.kt:370)
	at helium314.keyboard.latin.utils.CustomLayoutUtilsKt.checkLayout(CustomLayoutUtils.kt:88)
	at helium314.keyboard.latin.utils.CustomLayoutUtilsKt.editCustomLayout$lambda$23(CustomLayoutUtils.kt:190)
	at helium314.keyboard.latin.utils.CustomLayoutUtilsKt.$r8$lambda$v_xZ-D_ypZMa5XCv9wLuyd7erqI(CustomLayoutUtils.kt:0)
	at helium314.keyboard.latin.utils.CustomLayoutUtilsKt$$ExternalSyntheticLambda0.onClick(R8$$SyntheticClass:0)
	at androidx.appcompat.app.AlertController$ButtonHandler.handleMessage(AlertController.java:167)
	at android.os.Handler.dispatchMessage(Handler.java:106)
	at android.os.Looper.loopOnce(Looper.java:201)
	at android.os.Looper.loop(Looper.java:288)
	at android.app.ActivityThread.main(ActivityThread.java:7918)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)

Use case
I want to change case of the current word like described here #599 (comment). To do that, I want to select the word, then press shift with this key { "$": "multi_text_key", "codePoints": [-34, -11], "label": "U" }