refactor: make AICoreService tenant-agnostic & DI-friendly

[Schmarvinius]

Scope: API redesign of AICoreService + downstream wiring; biggest behavioral change.

• Drop tenantId parameter everywhere in the public API:
• resourceGroupForTenant(String) → resourceGroup() (impl reads tenant via TenantProviderService).
• Remove isMultiTenancyEnabled() from the interface (consume cds.multiTenancy.* standard CAP property internally instead of custom flag).
• Remove getRetry() from the interface (move to internal/abstract base or expose differently); javadoc "a bit unexpected to find this here".
• Drop @link references to AICoreServiceImpl / MockAICoreServiceImpl from interface javadoc – an interface must not know its impls.
• Consider segregating into:
• resource-group / deployment management
• inference (provide ApiClient inferenceClient(ModelDeploymentSpec) shortcut, remove explicit resourceGroupId/deploymentId params).
• Make the service injectable via Spring (@Autowired AICoreService, optional @Qualifier for multi-model).
• Update RecommendationClientResolver, RecommendationConfiguration, FioriRecommendationHandler, README "Programmatic Usage" code samples accordingly.

[github-actions]

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