Support referencing custom types from other custom types in `schema.types`

[kahirokunn]

Feature Description

Problem Statement:

Currently, custom types defined in ResourceGraphDefinition.spec.schema.types cannot reference each other, preventing the division and reuse of types. For example, if BaseCluster attempts to reference KubeconfigRef as shown below, the build of the RGD fails with an error unknown type: KubeconfigRef.

schema:
  spec:
    baseCluster: BaseCluster
  types:
    BaseCluster:
      kubeconfigRef: KubeconfigRef | required=true
    KubeconfigRef:
      name: string | required=true
      namespace: string | required=true

The current workarounds are to either inline and copy-paste KubeconfigRef or to fall back to the object notation. However, the object notation tends to weaken structured schema/validation. This goes against the intention of defining structures with types and then enhancing safety by applying CEL validation or constraints via markers.

Proposed Solution:

Allowing references to other schema.types within schema.types would be beneficial.

Expected Benefits:

• types can be modularized and reused, leading to a more concise schema.
• Structuring using types makes validation/CEL targets clearer compared to the object notation, making it easier to enhance safety.

Alternatives Considered:

Additional Context:

Current error example (from kubectl describe rgd ...):

conditions:
  - lastTransitionTime: "2025-12-17T04:15:40Z"
    message: 'failed to build resourcegraphdefinition ''k0smotrondockercluster.cluster.example.com'':
      failed to build OpenAPI schema for instance: failed to build OpenAPI schema
      for instance: failed to build pre-defined types schema: unknown type: KubeconfigRef'
    observedGeneration: 2
    reason: InvalidResourceGraph
    status: "False"
    type: ResourceGraphAccepted
  - lastTransitionTime: "2025-12-17T04:15:40Z"
    message: 'failed to build resourcegraphdefinition ''k0smotrondockercluster.cluster.example.com'':
      failed to build OpenAPI schema for instance: failed to build OpenAPI schema
      for instance: failed to build pre-defined types schema: unknown type: KubeconfigRef'
    observedGeneration: 2
    reason: InvalidResourceGraph
    status: "False"
    type: Ready

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[jakobmoellerdev]

PTAL at #714 and check if that could solve your use case!

[kahirokunn]

Yes, that achieves it. Thank you!