Improve REST API test coverage

[keithmanville]

Some portions of the v1 REST API are not covered by tests.

Identify endpoints that are not tested or could benefit from additional testing. Write a user story to help design new tests and implement them in the appropriate test suite.

These endpoints have already been identified as having a testing gap:

• /api/v1/entrypoints/{id}/queues
• /api/v1/entrypoints/{id}/queues/{id}
• /api/v1/entrypoints/{id}/plugins
• /api/v1/entrypoints/{id}/plugins/{id}
• /api/v1/experiments/{id}/entrypoints
• /api/v1/experiments/{id}/entrypoints/{id}

Definition of done:

• New tests are implemented to improve coverage
• All new and existing tests pass