Omitting a member's template.spec crashes compose-model-replica instead of failing admission

[negz]

What happened?

A ModelDeployment that omits spec.template.spec.engines[].members[].template.spec passes admission, because neither the ModelDeployment nor the ModelReplica XRD marks it required. compose-model-replica then asserts on it:

modelplane/functions/compose-model-replica/function/backends/llmd.py

Lines 127 to 129 in 0f42957

	# The XRD types template.spec as optional, but a member with no spec
	# defines no pod to serve, so reaching here without one is malformed.
	assert member.template.spec is not None

The AssertionError surfaces as a composition-function failure with a stack trace rather than a condition naming the field, so a user gets no indication of what's wrong with their manifest. The comment concedes the mismatch instead of the API enforcing it.

The same assert is at base.py#L333, and the pod-template equivalent at native.py#L96. #406 adds a Grove backend that copies the pattern, so it's worth fixing before more backends inherit it.

Adding required: [spec] to the member template in both XRDs makes this an admission error naming the field, and makes the asserts true by construction, like the leader/worker ones a CEL rule already guarantees. It does tighten the API: a manifest accepted today would be rejected, though today it only gets as far as crashing.

How can we reproduce it?

1. Apply a ModelDeployment whose engine member has template: {} (no spec), with everything else valid.
2. The XR is admitted, and the ModelReplica's composition fails on the assert above.

What environment did it happen in?

Modelplane version: main (0f42957b)