anthropic/v1/models: has_more is set while first_id and last_id stay null, so a contract-conformant client cannot page

[moomindani]

Summary

GET /ai-gateway/anthropic/v1/models implements Anthropic's cursor pagination for
limit, after_id and has_more, but it always returns first_id: null and
last_id: null — including when has_more is true. The Models API contract defines
last_id as the cursor a client feeds back as after_id ("Last ID in the data list.
Can be used as the after_id for the next page."), so a client written against the
documented contract stops after the first page and silently sees a truncated model list.
Only a client that reaches into data[-1].id can walk the list.

Observed

Workspace with 9 advertised Anthropic models; ug 0.1.0+161.gf26daf3.

Request	Response
?limit=2	2 models, has_more: true, first_id: null, last_id: null
?limit=2&after_id=<data[-1].id>	the next 2 models, no overlap with page 1
walk with limit=2, cursor taken from data[-1].id	5 pages, 9 unique ids — matches the unpaged read
no query parameters	9 models, has_more: false

curl -s -H "Authorization: Bearer $(databricks auth token --profile <profile> | jq -r .access_token)" \
  "https://<workspace-host>/ai-gateway/anthropic/v1/models?limit=2" \
  | jq '{has_more, first_id, last_id, n: (.data|length)}'
# => { "has_more": true, "first_id": null, "last_id": null, "n": 2 }

Expected

When data is non-empty, first_id is the first id in data and last_id is the last,
per the contract, so after_id / before_id walks work without reading into data.

Why it matters for ug

ug's own discovery read this endpoint once with no limit and ignored has_more
(_get_anthropic_models_json in src/ucode/databricks.py), so on a workspace advertising
more models than the endpoint's default limit of 20, every model after the first page was
invisible to ug configure, the model pickers and discover_claude_models. I have a patch
that walks the pages; because last_id is null it has to take the cursor from
data[-1].id, which is why this is worth fixing on the gateway side as well.

Reference: List Models — https://platform.claude.com/docs/en/api/models/list