ActiveContext: add indexing_embedding_fields to Collection class

What does this MR do and why?

• Adds the indexing_embedding_fields method to Collection
  • this will replace the version-based Collection.current_embedding_fields when we switch to the new model design
• Updates the ActiveContext::Databases::Elasticsearch::Client to use both Collection.current_embedding_fields and Collection.indexing_embedding_fields` when adding the embedding fields to the query
  • see explanation comment in the code
• Adds tests for the MarkRepositoryAsReadyEventWorker, which makes use of the current_embedding_fields
  • this worker is not yet using the new indexing_embedding_fields, but we need to add tests around the embedding fields in preparation for ActiveContext: integrate new model design into ... (!222417 - merged)

Step-by-step changes summary

MR	Status
Introduce the new hash/object-based models	✅
Add indexing_embedding_fields to Collection class	This MR
Add embeddings_with_model_redesign preprocessor	Ready for review
Integrate model redesign into Code Embeddings pipeline	Pending

References

• Issue: [ActiveContext] Redesign how models are referen... (#588847)
• MR introducing the hash/object-based model design: ActiveContext embedding model redesign (!222416 - merged)

Screenshots or screen recordings

N/A

How to set up and validate locally

Since we can be sure that indexing_embedding_fields is still empty in production, and that it's not yet used anywhere outside of the ActiveContext::Databases::Elasticsearch::Client.add_source_fields method, the unit tests should cover the validation.

You may validate in the console that indexing_embedding_fields is indeed an empty array if current_indexing_embedding_model are next_indexing_embedding_model model metadatas are both nil.

1. Set the metadata values:

   Ai::ActiveContext::Collections::Code.collection_record.update_metadata!(current_indexing_embedding_model: nil, next_indexing_embedding_model: nil)

2. indexing_embedding_fields should return an empty array

   Ai::ActiveContext::Collections::Code.indexing_embedding_fields
   => []

MR acceptance checklist

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Related to #588847