fix: handle null Xiaomi token plan responses safely

[DimazzzZ]

Summary

• fix Xiaomi token plan parsing when the API returns null for data, monthUsage, or items
• extract Xiaomi response parsing into a dedicated XiaomiResponseParser
• add focused tests for parser and provider behavior

Problem

Xiaomi token plan refresh could throw a ClassCastException when the platform returned JsonNull instead of the expected array/object structure for stopped or expired plans:

java.lang.ClassCastException: class com.google.gson.JsonNull cannot be cast to class com.google.gson.JsonArray
    at com.google.gson.JsonObject.getAsJsonArray(JsonObject.java:211)
    at ...XiaomiProviderClient.fetchTokenPlanUsage$lambda$1(XiaomiProviderClient.kt:139)

Changes

• replace unsafe getAsJsonArray() / getAsJsonObject() calls with null-safe accessor extensions (getArrayOrNull, getObjectOrNull, getStringOrNull, etc.)
• move Xiaomi response mapping logic into XiaomiResponseParser
• preserve expected inactive-plan behavior when usage data is absent (0 tokens, "inactive" status)
• expand Xiaomi provider tests
• add dedicated parser tests covering success, null/missing fields, and error envelopes

Validation

• ./gradlew test --tests "org.zhavoronkov.tokenpulse.provider.xiaomi.*" — all tests pass
• Xiaomi package line coverage: 95.0% (114/120)