server : support multiple model aliases via comma-separated --alias (#19926)

server : support multiple model aliases via comma-separated --alias (#19926)

* server : support multiple model aliases via comma-separated --alias

* server : update --alias description and regenerate docs

* server : multiple model aliases and tags

- address review feedback from ngxson
- --alias accepts comma-separated values (std::set, no duplicates)
- --tags for informational metadata (not used for routing)
- aliases resolve transparently in router via get_meta/has_model
- /v1/models exposes aliases and tags fields

* regenerate docs

* nits

* server : use first alias as model_name for backward compat

address review feedback from ngxson

* server : add single-model test for aliases and tags