feat: move public APIs to dedicated api packages

[Schmarvinius]

Scope: structure-only refactor, no behavior change.

• Move to com.sap.cds.feature.aicore.api:
• AICoreService, ModelDeploymentSpec, RptModelSpec (or whatever is intended public)
• Move to com.sap.cds.feature.recommendation.api:
• RecommendationClient, RecommendationClientResolver, RptInferenceClient
• Update package-info.java / module-info / META-INF/services if any.
• Keep impls in core/internal package.

[github-actions]

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