[MSDK-3955] Expose mandatoryLabel in TCF2Settings Flutter bridge for PUR compliance

[uc-brunosilva]

Summary

• Adds mandatoryLabel: String (default: "Mandatory") to TCF2Settings in the Flutter bridge — missed in the original PR [MSDK-3955] Expose ConsentOrPay settings in Flutter bridge for PUR co… #171 that exposed consentOrPay
• Both fields were added to the mobile-sdk TCF2Settings in 2.27.0; only consentOrPay was bridged

Changes

• lib/src/model/tcf2_settings.dart — added field declaration, constructor param, ==, hashCode
• lib/src/internal/serializer/tcf2_settings_serializer.dart — deserialize from value['mandatoryLabel'] with "Mandatory" fallback
• android/.../CMPDataSerializer.kt — serialize mandatoryLabel
• ios/Classes/Serializer/CMPDataSerializer.swift — serialize mandatoryLabel

Why it matters

Flutter developers building a custom TCF second layer via getCmpData() need mandatoryLabel to render the mandatory badge on purposes/special features that have no toggle under the ConsentOrPay flow. Without it they would have to hardcode "Mandatory", which breaks when the value is overridden via the app_PUR_MANDATORY_LABEL_TEXT translation key.

Test plan

• All 64 Flutter unit tests pass (flutter test)