Expose JA4 through a standard request interface #283

Description

Expose the incoming request’s JA4 fingerprint through a standard EdgeZero interface. The existing Fastly raw-request hook lets applications define their own extension type, but does not provide a neutral JA4 type or cross-adapter API.

Steps to reproduce

Inspection steps (no provider request executed):

1. Read Fastly raw-request hook guidance.
2. Observe that the example defines an application-local Ja4(String) and calls the raw Fastly SDK.
3. Look for an equivalent neutral accessor and adapter population contract; none is present on main.

Expected behavior

Handlers and middleware can read optional JA4 metadata through one documented interface, with explicit unavailable behavior where a provider cannot supply it.

Impact

Applications duplicate provider-specific extraction and cannot consistently consume JA4 across adapters.

Proposed fix

Define a neutral optional JA4 request metadata/accessor contract and populate it from supported provider signals. Preserve provenance and absence semantics; do not infer a fingerprint from ordinary client-supplied headers.

Done when

• Add and document the standard JA4 interface.
• Populate it on adapters whose trusted provider APIs expose JA4.
• Return explicit absence/unsupported behavior on other adapters.
• Cover present, absent and unsupported metadata paths with adapter/core tests.

Related

PR #306 supplies the Fastly extension hook only; it is related, not a closing implementation.