Handle oversized HEAD response metadata

[ChristianPavilonis]

Summary

• Allow HEAD responses to preserve oversized Content-Length metadata without triggering the buffered-body safety limit.
• Keep the existing 10 MiB limit intact for buffered responses that can allocate origin body bytes in WASM memory.

Changes

File	Change
crates/trusted-server-adapter-fastly/src/platform.rs	Track whether the request expects a response body, return an empty body for HEAD responses, and add oversized HEAD/GET regression coverage.

Closes

Closes #950

Test plan

• cargo test-fastly && cargo test-axum
• cargo clippy-fastly && cargo clippy-axum
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run
• JS format: cd crates/trusted-server-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: cargo test-fastly; cargo clippy-fastly

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