llm-gateway-stack: demo UI key check passes when the gateway uses an external API-key Secret

[kristinapathak]

Describe the bug

deploy/helm/llm-gateway-stack/llm-gateway-stack/templates/secret-demo-ui.yaml checks that demoUiApiKey is registered by looking for a matching demo-ui hash in .Values.apiKeys. When apiKeysSecret.create is false, the gateway reads a Secret the chart does not manage and ignores .Values.apiKeys. The check can then pass even though the external Secret has no demo-ui entry. The chart creates the demo-ui-api-key Secret, and the gateway rejects that key.

Steps or code to reproduce bug

1. Set apiKeysSecret.create: false and provide your own key-file Secret without a demo-ui entry.
2. Set demoUiApiKey, and add its demo-ui hash to apiKeys.
3. Install the chart. The render succeeds and demo-ui-api-key is created.
4. Call the gateway with the key from demo-ui-api-key. It returns 401.

Expected behavior

Either the chart rejects demoUiApiKey when apiKeysSecret.create is false, or it documents that the external Secret must contain the demo-ui hash, and the demo UI setup verifies that.

Additional context

Introduced with the demo UI key in #2322. Found during review of #2331, which does not change this chart.