Custom-action endpoint that runs each query of the request against all open libraries.

The URL form libraries:query follows Google's API Improvement Proposal 136 — Custom methods: custom (non-standard CRUD) methods are spelled <resource>:<verb> with a literal colon and no separating slash. Quoting AIP-136:

Custom methods should use HTTP POST if they have side effects, or HTTP GET if they are side-effect free. ... Custom methods should generally take the form: https://service.googleapis.com/v1/resources/{id}:verb. ... For batch and collection-level custom methods, the colon is appended directly to the collection name: https://.../v1/resources:batchGet.

Practical reason for the colon here: the sibling LibraryResource is mounted at @Path("libraries/{id}"). A libraries/query sibling would lose JAX-RS path-specificity arbitration because the per-library path matches {id}=query with more literal characters — the request would dispatch to LibraryResource and return 405. The colon variant has no slash and therefore no collision.