Building metadata with non-existent subgraphs succeeds without warning or errors

When building the metadata and defining relationships in the HML, if you reference a non-existent subgraph the metadata will still build and then the relationships won't appear.

```
---
kind: Relationship
version: v1
definition:
name: HelloWorld
sourceType: Users
target:
command:
name: Hello
subgraph: some_subgraph_that_doesnt_exist // This silently fails
mapping:
- source:
fieldPath:
- fieldName: name
target:
argument: name
```

It would be good to validate the metadata and warn the user if a subgraph doesn't exist.

No files, tests, or entry points are named. Start by locating the metadata-building path for Relationship definitions and reproduce the HML example with a nonexistent subgraph; done means the build reports a warning or error instead of silently omitting the relationship.

Written by the indexing model from the issue text.