feat: add optional displayName to kagent ModelConfig entries

[marians]

What

Adds an optional displayName to each agents.models[] entry. When set, it is rendered as the ui.giantswarm.io/display-name annotation on the generated kagent ModelConfig CR so the UI can show a friendly label. When omitted, no annotations: block is emitted.

This also adds the previously-missing agents.models schema to both values.schema.json files, so model entries are now validated.

Changes

• Template (agentic-platform-connectivity/templates/kagent/agents/modelconfigs.yaml): render the annotation only when displayName is present, via a {{- with .displayName }} guard.
• Schema (both values.schema.json, kept in sync): new agents.models array schema — required name/provider/model/apiKeySecret, optional displayName/apiKeySecretKey, additionalProperties: false.
• Values examples (both values.yaml): added the commented displayName example line.
• CHANGELOG: entry under ### Added.

Verification

helm template on the connectivity chart:

• with displayName → ui.giantswarm.io/display-name: "…" rendered under metadata.annotations.
• without displayName → no annotations: block, no error.
• unknown field → rejected by schema (additional properties '…' not allowed), confirming validation is enforced.

helm lint passes on both charts.

Checklist

• Update changelog in CHANGELOG.md.
• Make sure values.yaml and values.schema.json are valid.