[recommendations] Support @UI.RecommendationState annotation

[Schmarvinius]

Context

The Node.js counterpart (@cap-js/ai) supports dynamic @UI.RecommendationState expressions that are evaluated per-row at runtime. This allows conditionally disabling recommendations for specific fields based on the current row's data (e.g., disable genre recommendation once a genre is already set).
Currently, the Java cds-feature-recommendations module only supports the static form @UI.RecommendationState: 0 (completely disables a field at compile time).

Example

entity Books : cuid, managed {
  title    : String;
  genre    : Association to Genres;
  currency : Association to Currencies;
}
// Static (already supported): always disabled
annotate Books with { genre @UI.RecommendationState: 0 };
// Dynamic (not yet supported): disabled when genre is already filled
annotate Books with { genre @UI.RecommendationState: { $edmJson: { $If: [{ $Ne: [{ $Path: 'genre_ID' }, null] }, 0, 2] } } };

Approach (from Node.js reference)

1. During context row retrieval, detect elements with @UI.RecommendationState containing an expression (xpr)
2. Add them as computed columns to the SELECT query: { xpr: [...], as: 'SAP_RECOMMENDATION_STATE_{field}', cast: Integer }
3. After fetching the draft row, evaluate the result per-field:

• If SAP_RECOMMENDATION_STATE_{field} == 0 → skip prediction for that field (don't set [PREDICT])
• Otherwise → proceed as normal

4. Exclude fields with disabled state from the final SAP_Recommendations response

Acceptance Criteria

• Dynamic expressions in @UI.RecommendationState are evaluated per-row
• Fields with state 0 (disabled) are excluded from prediction and response
• Static @UI.RecommendationState: 0 continues to work (no regression)
• Unit test covering dynamic expression evaluation
• Integration test with a draft entity using conditional recommendation state

[Schmarvinius]

• Allow the use of static RecommendationState (0 or 1)
• Allow the use of dynamic RecommendationState (expression)