Improve Engine.check argument errors

[KirtiRamchandani]

Problem

• Passing a non-string value to Engine.check currently surfaces Neon's generic downcast error, which does not identify which request argument is wrong.

Root cause

• The binding layer directly downcasts the url, source_url, and request_type arguments without adding API-level context.

Solution

• Add a small string argument helper that reports the expected argument name.
• Use it for Engine.check request arguments.
• Add JS binding coverage for invalid url, source_url, and request_type values.

Tests run

• cargo fmt --all -- --check
• cargo check -p adblock-rs
• cargo test -p adblock --lib resources::resource_storage -- --nocapture
• npm install --ignore-scripts
• npm run build
• npm run test:js
• cargo test -p adblock-rs --no-default-features
• git diff --check

Fixes #182