Add temporary JA4/TLS debug endpoint

[prk-Jr]

Summary

• Adds a temporary GET /_ts/debug/ja4 endpoint to the Fastly adapter to surface JA4 TLS fingerprint, H2 fingerprint, cipher suite, TLS version, user-agent, and selected Client Hints for browser fingerprint evaluation
• Keeps the endpoint response readable when Fastly omits individual TLS or Client Hint values by using explicit fallback text
• Adds route-level regression coverage so the adapter test suite exercises both the response builder and the /_ts/debug/ja4 router path

Changes

File	Change
crates/trusted-server-adapter-fastly/src/main.rs	Added build_ja4_debug_response, registered GET /_ts/debug/ja4, and added unit coverage for fallback response fields
crates/trusted-server-adapter-fastly/src/route_tests.rs	Added a route-level test covering /_ts/debug/ja4 dispatch, content type, and fallback body values

Closes

Closes #645

Test plan

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• GitHub Actions checks green for cargo test, vitest, integration tests, browser integration tests, format-docs, format-typescript, cargo fmt, and CodeQL
• Manual testing via fastly compute serve

Checklist

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