[codex] Harden share plugin release candidate

[AbassHammed]

Summary

This release-candidate branch hardens tauri-plugin-vnidrop-share before publishing and adds regression coverage plus a runnable Tauri example app.

What changed

• Added JS, Rust, Android, and iOS validation for share payload limits, malformed URLs, URL schemes, file counts, file sizes, file names, and MIME metadata.
• Removed cleanup from the default permission set so temporary-file cleanup remains explicitly opt-in.
• Added Rust integration tests, Vitest coverage, Android JVM tests, and Swift validation tests with local native stubs.
• Added a Tauri example app that registers the plugin, includes the required permissions, and exercises share/cleanup flows.
• Documented Android Plugin not found troubleshooting and release validation commands.

Validation

• cargo test
• bun run build && bun run test && bun run test:types && bun audit
• cd android && gradle testDebugUnitTest
• cd ios && VNIDROP_SHARE_USE_TAURI_STUB=1 swift test
• cd examples/tauri-app && npm run build && npm audit
• cd examples/tauri-app/src-tauri && cargo check
• cargo audit for root and example lockfiles: no active vulnerabilities; remaining output is warning-only transitive Tauri/Linux GTK stack advisories.
• Android dev reinstall was tested on Medium_Phone_API_36.0; the previous vnidrop-share.can_share not allowed. Plugin not found error was not present after rebuild.