refactor(recommendations): RecommendationClient API + handler/builder cleanup

[Schmarvinius]

Scope: API redesign + multiple correctness/quality fixes in one module. Could split if too big.

• Redesign RecommendationClient.predict to separate prediction row from context rows:
  List predict(CdsData predictionRow, List contextRows,
  List predictionColumns, String indexColumn);
• Add public constant RecommendationClient.PREDICT = "[PREDICT]"; use everywhere (RptInferenceClient, MockRecommendationClient, RecommendationContextBuilder).
• Decouple from RPT-1 specifics: move compound-key handling, SYNTHETIC_KEY_COLUMN assembly, and "row assembly" out of RecommendationContextBuilder into RptInferenceClient.
• Added https://github.tools.sap/cap/cds-calesi/issues/1187: Register RecommendationClient as a CAP service so apps can override its on-handler (could be tracked as follow-up if too large).
• Drop @FunctionalInterface on RecommendationClientResolver.
• Rename RptInferenceClient.api → rpt. Extract a toSdkRow(...) helper method.
• Replace MANAGED_FIELDS set with annotation-driven exclusion: @Core.Computed, @readonly.
• FioriRecommendationHandler fixes:
  • Use a reliable IsActiveEntity check (current substring match is unsafe; only works when IsActiveEntity is selected).
  • Skip context query early if predictionColumns is empty.
  • Predict only columns where currentRow.get(c) == null.
  • Resolve PersistenceService via DI, not lookup.
  • Extract "SAP_Recommendations" as constant.
  • Reduce log level from info to trace (or drop).
• RecommendationContextBuilder fixes:
  • Throw / suppress predictions when key cannot be determined (drop orElse("ID")).
  • Use Set selectColumns to dedupe.
  • Drop the unnecessary ORDER BY (or document its purpose).
  • Verify whether the prediction row is implicitly excluded; otherwise exclude it explicitly. Add tests.
  • Use instanceof CdsSimpleType st && SUPPORTED_CONTEXT_TYPES.contains(st.getType()).
  • Replace '\0' synthetic-key separator with a documented & well-tested strategy.
  • Don't react on @cds.odata.valuelist : false.

[github-actions]

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