CBOR `@CborLabel` with negative label value decodes incorrectly

[ardune]

Describe the bug
When using negative values for @CborLabel when combined with preferCborLabelsOverNames decoding fails

To Reproduce
Quick Test file

package kotlinx.serialization.cbor

import kotlinx.serialization.*
import kotlin.test.*

class NegativeLabelsOverNamesTest {
    private val withLabel = Cbor {
        preferCborLabelsOverNames = true
        useDefiniteLengthEncoding = true
    }

    @Test
    fun withNegativeTwoLabel() {
        val target = WithNegativeTwo(
            3,
            true
        )
        // a2010321f5
        /**
         * A2    # map(2)
         *    01 # unsigned(1)
         *    03 # unsigned(3)
         *    21 # negative(1)
         *    F5 # primitive(21)
         */
        val bytes = withLabel.encodeToByteArray(target)

        // failure - kotlinx.serialization.cbor.internal.CborDecodingException: Expected an unsigned or negative integer, but found F5
        val decodeBack: WithNegativeTwo = withLabel.decodeFromByteArray(bytes)
        assertEquals(target, decodeBack)
    }

    @Test
    fun withNegativeOneLabel() {
        val target = WithNegativeOne(
            3,
            true
        )
        //a2010320f5
        /**
         * A2    # map(2)
         *    01 # unsigned(1)
         *    03 # unsigned(3)
         *    20 # negative(0)
         *    F5 # primitive(21)
         */
        val bytes = withLabel.encodeToByteArray(target)

        // failure - kotlinx.serialization.cbor.internal.CborDecodingException: CborLabel unknown: 0 for kotlinx.serialization.cbor.NegativeLabelsOverNamesTest.WithNegativeOne(id: kotlin.Int, cool: kotlin.Boolean?)
        val decodeBack: WithNegativeOne = withLabel.decodeFromByteArray(bytes)
        assertEquals(target, decodeBack)
    }

    @OptIn(ExperimentalSerializationApi::class)
    @Serializable
    data class WithNegativeTwo(
        @CborLabel(1)
        val id: Int,
        // critical bit: negative label causes failure to decode
        @CborLabel(-2)
        val cool: Boolean? = null,
    )

    @OptIn(ExperimentalSerializationApi::class)
    @Serializable
    data class WithNegativeOne(
        @CborLabel(1)
        val id: Int,
        // critical bit: negative label causes failure to decode
        @CborLabel(-1)
        val cool: Boolean? = null,
    )
}

Expected behavior
Tests should pass.

Fix
The fix I applied locally to 1.11.0 is attached as a patch (one liner):

kotlinx.serialization-3-41-26 PM.patch

This does change that line from:
e334d1c
Which was intended to make reading strictly follow the RFC - but since it only allowed positive numbers I just changed it to the generic 'readNumber' to handle both positive and negative.

Environment

• Kotlin version: 2.3.20
• Library version: 1.11.0
• Kotlin platforms: All
• Gradle version: 8.7

[vsartiushkin]

Thank you for the report!