Add test coverage for error mapping and tsjs helpers

[ChristianPavilonis]

Summary

• add exhaustive unit coverage for TrustedServerError::status_code() so each public variant is pinned to its expected HTTP response
• add direct tsjs formatting tests for unified and deferred script URL/tag helpers using computed hashes
• cover invalid UTF-8 cookie-header handling and simplify duplicate callback serde rename tests

Changes

File	Change
crates/trusted-server-core/src/error.rs	Added a table-driven test covering HTTP status code mapping for every TrustedServerError variant.
crates/trusted-server-core/src/tsjs.rs	Added direct unit tests for unified and deferred TS/JS script URL and tag helpers.
crates/trusted-server-core/src/cookies.rs	Added a unit test for invalid UTF-8 Cookie header handling in handle_request_cookies().
crates/trusted-server-core/src/models.rs	Consolidated duplicate callback serde rename tests into one focused test.

Closes

Closes #448
Closes #450
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:

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