Fix querying repository blobs in GraphQL

What does this MR do?

Three problems, not picked up while reviewing this MR: !58677 (merged)

First, the args argument to resolver_complexity doesn't implement fetch, so any queries are currently broken.

Second, ::Blob type doesn't implement a couple of fields found in TreeEntry, and BlobType depends on them. So if you query for them from the new Blob endpoint, the query fails.

Third, sharing the GraphQL Blob type between the two endpoints is not practical.

Fix all three problems before a release, and introduce a feature spec to prevent the first two from recurring.

Does this MR meet the acceptance criteria?

Conformity

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  • I have not included a changelog entry because I'm fixing a bug introduced in the same release (13.11)
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