VC issuing fails if credentialSubject is the same as the issuer

Now that we have one named graph per resource, the `credentialSubject` property of VCs can be stored without needing to use special properties.

However if the `credentialSubject` ID is the same as the issuer, like on the "Non-linked chain is invalid" test (verifiable-credentials.test.ts) then we have a problem.

This is what we want to create in this test:

```
{
'@context': [
'https://www.w3.org/ns/credentials/v2',
{
as: 'https://www.w3.org/ns/activitystreams#',
apods: 'http://activitypods.org/ns/core#',
acl: 'http://www.w3.org/ns/auth/acl#',
dc: 'http://purl.org/dc/terms/',
sec: 'https://w3id.org/security#'
}
],
type: [ 'VerifiableCredential' ],
issuer: 'http://localhost:3000/craig/webid',
name: 'Second capability',
credentialSubject: {
id: 'http://localhost:3000/craig/webid',
name: 'This is an additional field',
description: 'A transferable capability.'
}
}
```

This is stored like this on Fuseki, in the `http://localhost:3000/craig/7221ae4a-1ac8-4019-a31f-d7323824a47a` named graph:

```ttl

a ;

"2025-11-12T08:35:58.418Z"^^;

;

"2025-11-12T08:35:58.418Z"^^;
"Second capability";

;

.

"A transferable capability.";
"This is an additional field" .
```

When we retrieve the VC, it looks like this:

```
{
'@context': [
'https://www.w3.org/ns/credentials/v2',
{
as: 'https://www.w3.org/ns/activitystreams#',
apods: 'http://activitypods.org/ns/core#',
acl: 'http://www.w3.org/ns/auth/acl#',
dc: 'http://purl.org/dc/terms/',
sec: 'https://w3id.org/security#'
}
],
id: 'http://localhost:3000/craig/7221ae4a-1ac8-4019-a31f-d7323824a47a',
type: 'VerifiableCredential',
'dc:created': {
type: 'http://www.w3.org/2001/XMLSchema#dateTime',
'@value': '2025-11-12T08:35:58.418Z'
},
'dc:creator': {
id: 'http://localhost:3000/craig/webid',
description: 'A transferable capability.',
name: 'This is an additional field'
},
'dc:modified': {
type: 'http://www.w3.org/2001/XMLSchema#dateTime',
'@value': '2025-11-12T08:35:58.418Z'
},
name: 'Second capability',
credentialSubject: 'http://localhost:3000/craig/webid',
issuer: 'http://localhost:3000/craig/webid'
}
```

The `credentialSubject` is not embedded because we decided not to do embedding when there are more than one corresponding property. If we did embedding above, the issuer would also include the `name` and `description` properties, which would be wrong.

Note: The embedding works fine if the issuer is not the same as the credentialSubject.

Any idea @Laurin-W ? I'm going to leave it like this on my branch `uuids-for-containers-and-collections` even if tests fails.

Start with the “Non-linked chain is invalid” test in verifiable-credentials.test.ts and reproduce the failing VC issuance and retrieval case against Fuseki. Trace how issuer and credentialSubject are serialized and embedded when they share an ID; done means the test passes, credentialSubject remains represented correctly, and issuer properties are not incorrectly included.

Written by the indexing model from the issue text.