Distinguish unresolved from resolved-empty required resources

先閱讀 request.get_required_resource 和 get_required_resources API 及其 docstring，接著比較相應的 require_schema/get_required_schema 行為。定義支援的 unresolved、absent 和 present 情況，並透過測試確認所選擇的 API 形式；完成的標準是呼叫端無需重複 membership check 即可區分這些狀態。

What problem are you facing?

request.get_required_resource (and get_required_resources) returns None/[] in two semantically different cases: the requirement hasn't been resolved by Crossplane yet, and the requirement resolved but matched nothing. The only way to tell them apart is to also check name in req.required_resources, which the get_required_resources docstring documents as a caveat the caller must handle by hand.

This distinction matters whenever a function gates on an external resource. "Not resolved yet" is transient — wait and let Crossplane re-call. "Resolved, matched nothing" is a real state — the resource was deleted, or its reference is a typo — and usually wants a different response (e.g. a warning, or holding steady rather than waiting). Functions end up reimplementing the same in-then-get dance. We hit it twice in one project: once gating a hydration Job on an auth Secret, and once gating replica placement on a referenced cache being resolvable.

How could this Function help solve your problem?

A small helper that classifies a required resource into the three states, so callers don't each re-derive it from the in check. Roughly:

class Resolution(enum.Enum):
    UNRESOLVED = "unresolved"  # Crossplane hasn't fetched it yet (key absent)
    ABSENT     = "absent"      # resolved, but nothing matched
    PRESENT    = "present"     # resolved and found

def resolve_required(req, name) -> tuple[Resolution, dict | None]: ...

A list variant (and possibly the same for require_schema/get_required_schema, which has the analogous distinction) would round it out. Naming/placement and whether this is an enum vs. a small result type are open — happy to send a PR once there's a preferred shape.