Align Automatic Migrations connector resolution with Assistant and Agent Builder

[logeekal]

Summary

Aligns Automatic Migrations (SIEM Migrations) with the same connector resolution scheme used by Assistant and Agent Builder.

After #258530, Automatic Migrations was still using the old connector loading path (loadAllActions from triggers-actions-ui), which returns preconfigured connector YAML keys (e.g., Anthropic-Claude-Opus-4-6) as connector IDs. This meant the connectorId passed to naturalLanguageToEsql was a model_id (old scheme) instead of the inference_id, resulting in a Saved object [action/.anthropic-claude-4.6-opus-chat_completion] not found error.

This PR switches Automatic Migrations to use the inference plugin's connector resolution (same as Assistant and Agent Builder), which returns inference_id-based connector IDs.

Changes

• Frontend: Replaced useAIConnectors / loadAiConnectors with useLoadConnectors / loadConnectorsForFeature from @kbn/inference-connectors (scoped to featureId: 'siem_migrations')
• Backend: Replaced actionsClient.get() with inferenceClient.getConnectorById() in rules and dashboards start routes
• @kbn/inference-connectors: Extracted loadConnectorsForFeature as a reusable stateless function from useLoadConnectors

Test plan

• Select a preconfigured inference connector in the SIEM Migrations onboarding card
• Start a rule migration — should succeed without "saved object not found" error
• Start a dashboard migration — same
• Verify connector selection persists across modal reopens (via localStorage)

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[elasticmachine]

⏳ Build in-progress, with failures

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• Commit: 233126d

Failed CI Steps

• FTR Configs #31
• FTR Configs #31
• FTR Configs #178
• FTR Configs #178

Test Failures

• [job] [logs] FTR Configs #31 / @ess @serverless SecuritySolution Automatic Migrations @ess @serverless SecuritySolution Automatic Rule Migrations Start Migration error scenarios should reject if connector_id is incorrect
• [job] [logs] FTR Configs #178 / @ess @serverless SecuritySolution Automatic Migrations @ess @serverless SecuritySolution Automatic Rule Migrations Start Migration error scenarios should reject if connector_id is incorrect
• [job] [logs] FTR Configs #31 / @ess @serverless SecuritySolution Automatic Migrations @ess @serverless SecuritySolution Automatic Rule Migrations Start Migration error scenarios should reject if connector_id is incorrect
• [job] [logs] FTR Configs #178 / @ess @serverless SecuritySolution Automatic Migrations @ess @serverless SecuritySolution Automatic Rule Migrations Start Migration error scenarios should reject if connector_id is incorrect

History