Distinguish unresolved from resolved-empty required resources

### 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:

```python
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.

Start by reading the request.get_required_resource and get_required_resources APIs and their docstrings, then compare the analogous require_schema/get_required_schema behavior. Define the supported unresolved, absent, and present cases and confirm the chosen API shape with tests; done means callers can distinguish those states without duplicating membership checks.

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