Use valid image proxy content type fallback

[ChristianPavilonis]

Summary

• Replaces the invalid image/* response Content-Type fallback with application/octet-stream.
• Covers both proxy response finalization paths and updates the first-party proxy guide.

Changes

File	Change
crates/trusted-server-core/src/proxy.rs	Added a shared image fallback content-type constant, used it in normal and streaming finalization paths, and updated tests.
docs/guide/first-party-proxy.md	Updated image handling docs to describe the valid fallback and trigger behavior.

Closes

Closes #429

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: cargo test -p trusted-server-core proxy; cargo test -p trusted-server-core

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