Update Agent Builder docs for 9.4+ inference apps

[charlotte-hoblik]

1. Update models.md for the 9.4+ Model management UX

Currently models.md frames model setup around AI connectors. In 9.4+ that flow has been replaced by the Elastic inference and External inference apps under Stack Management → Model management, with models and endpoints as the primary nouns.

2. Add an Inference endpoint entry to glossary.md

The term is now user-facing in AB across several surfaces: the Elastic inference and External inference apps, the "Agent Builder inference endpoint configuration" section in Feature settings, the cluster-privilege reference in mcp-server.md, and the Elasticsearch _inference API.

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[lcawl]

Interestingly, in 9.4+ the "Inference Endpoints" UI has been replaced by Elastic inference and External inference apps. The latter app still has the term "inference endpoint" (but generally they both focus on "models" and "endpoints"). In the "model management" apps context, connectors are no longer something they want to call out. Where the "inference endpoint" terminology still persists, however is the API docs, e.g. https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-inference-put Hope this info from my recent 9.4 work is helpful!