fix: preserve shared file display names

[Airsaid]

Summary

• Write ShareToolkit-generated Android and iOS share files under per-file UUID directories so receiving apps see the safe file name as the final path segment.
• Update Android share-cache cleanup to remove expired top-level cache children recursively while preserving direct sharing for files already inside toolkit_share.
• Add Android host coverage for unique share directories, recursive cleanup, legacy file cleanup, nested cache detection, and the direct-cache cleanup ordering risk.

Affected modules

• toolkit Android share implementation and Android host tests.
• toolkit iOS share temp file writing.

Tests

• rtk ./gradlew :toolkit:testAndroidHostTest
• rtk ./gradlew :toolkit:compileKotlinIosSimulatorArm64
• rtk ./gradlew :toolkit:lint --no-parallel