Polish Kotlin runtime hardening

[papi-ux]

Summary

• harden NvHTTP URL construction and request targeting around known NVHTTP paths
• tighten Android entry points for external display notification and .art URI handling
• keep Kotlin runtime cleanup focused on null-safety and Java interop warnings
• add guard tests for cache path escapes, external display receiver contracts, and .art URI validation

Test Plan

• ./gradlew -PnovaAbis=x86_64 compileNonRoot_gameDebugKotlin --console=plain --rerun-tasks
• ./gradlew -PnovaAbis=x86_64 testNonRoot_gameDebugUnitTest --tests 'com.papi.nova.utils.CacheHelperTest' --tests 'com.papi.nova.utils.KotlinExternalDisplayUiMigrationTest' --tests 'com.papi.nova.KotlinLaunchHelpersMigrationTest' --tests 'com.papi.nova.nvstream.KotlinNvstreamRuntimeMigrationTest' --tests 'com.papi.nova.nvstream.http.NvHttpServerInfoParsingTest' --console=plain\n- ./gradlew -PnovaAbis=x86_64 testNonRoot_gameDebugUnitTest --console=plain\n- ./gradlew -PnovaAbis=x86_64 assembleNonRoot_gameDebug --console=plain\n- ./gradlew -PnovaAbis=x86_64 -PlintFailOnError=true lintNonRoot_gameDebug --console=plain\n- git diff --check