fix: purely unique swagger contracts are incomplete

Seen in oss-diff, there are missing common schemas that the individual pieces are referring to that are no longer being included.

Possible solution is to migrate those schemas out of the `_schemas.yml` and into `cloud.yml` and `oss.yml` so the `-diff.yml` contracts are generated correctly, or do another sed replacement with the shared/diff ones.

Compare the missing schemas referenced by the purely unique contracts in oss-diff with _schemas.yml, cloud.yml, and oss.yml. Start by tracing how the -diff.yml contracts are generated and determine whether shared schemas should move into the individual specifications or be handled by the existing sed replacement. Done means the generated diff contracts include every referenced common schema.

Written by the indexing model from the issue text.