🌊 Streams: Use shared EIS callout for pattern generation

[flash1293]

Based on #220782

Fixes https://github.com/elastic/obs-ai-assistant-team/issues/256

To test:

• Make sure you have a trial or enterprise license
• Either
  • Start EIS as described in Darios email "AI for everyone"
  • Add the config snippet to your kibana.dev.yml
  • Start es with -E xpack.inference.elastic.url=http://localhost:8443/
• Or
  • Mock out x-pack/platform/plugins/shared/observability_ai_assistant/public/utils/get_elastic_managed_llm_connector.ts and just return true to simulate the current connector being EIS
• Make sure there is at least one sample document to "Generate patterns" is enabled
• The callout should show once, but not come back after dismissed

[elasticmachine]

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[flash1293]

Closing since #223554 did the job