Harden inline creative rendering

[prk-Jr]

Summary

• Harden the core requestAds renderer so untrusted inline creatives cannot escape the iframe sandbox or execute retained dangerous markup.
• Fail closed on malformed or sanitized-away creatives with structured rejection metadata while avoiding raw creative HTML in logs.
• Add regression coverage for sandbox permissions, dangerous URI/style payloads, malformed creatives, and accepted safe markup.

Changes

File	Change
crates/js/lib/package.json	Add dompurify as a runtime dependency for core creative sanitization.
crates/js/lib/package-lock.json	Lock the new DOMPurify dependency and its transitive package metadata.
crates/js/lib/src/core/render.ts	Sanitize untrusted creative HTML, reject malformed or dangerous markup, and tighten iframe sandbox permissions.
crates/js/lib/src/core/request.ts	Route every inline creative through the sanitizer before srcdoc injection and add structured render/rejection logging metadata.
crates/js/lib/test/core/render.test.ts	Cover sandbox tokens, accepted safe markup, rejected dangerous URI/style payloads, malformed creatives, and empty sanitization results.
crates/js/lib/test/core/request.test.ts	Cover safe request-path rendering plus fail-closed behavior for dangerous, malformed, and empty creatives without logging raw HTML.

Closes

Closes #401

Test plan

• cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --bin trusted-server-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: cd crates/js/lib && npm run build

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed