Clarify cookie header tests and remove duplicate callback serde test

[ChristianPavilonis]

Summary

• add a focused unit test for the invalid UTF-8 Cookie header path in handle_request_cookies
• rename the existing malformed-cookie test so it matches the behavior it actually covers
• remove a redundant callback serde rename test while preserving direct and broader deserialization coverage

Changes

File	Change
crates/trusted-server-core/src/cookies.rs	Renamed the malformed cookie-string test and added a new invalid UTF-8 cookie-header test using HeaderValue::from_bytes(...) to exercise the real InvalidHeaderValue branch.
crates/trusted-server-core/src/models.rs	Removed the duplicate callback serde rename test, keeping the existing focused direct test and broader callback deserialization coverage.

Closes

Closes #454
Closes #456

Test plan

• cargo test --workspace
• cargo clippy --workspace --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 --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: Rust-only test cleanup; no production behavior changes expected

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