OAK-12353: Group Elasticsearch dynamic boost nested documents by boost score

[fabriziofortino]

Summary

Dynamic boost properties (e.g. predictedTagsDynamicBoost) are indexed in Elasticsearch
as nested documents, with one nested document per value:

"predictedTagsDynamicBoost": [
  { "value": "Replacement Cost", "boost": 1 },
  { "value": "Theft", "boost": 1 },
  { "value": "Alberta", "boost": 1 },
  { "value": "GENERAL INSURANCE COMPANY", "boost": 0.988 }
]

For properties with many values sharing the same boost score, this creates a large
number of nested documents, which is expensive to index and store.

This change groups values that share the same boost score into a single nested
document, with value holding an array instead of a scalar:

"predictedTagsDynamicBoost": [
  { "value": ["Replacement Cost", "Theft", "Alberta"], "boost": 1 },
  { "value": "GENERAL INSURANCE COMPANY", "boost": 0.988 }
]

No mapping or query changes were needed: value is a plain analyzed text field,
which Elasticsearch accepts as an array natively, and the existing nested match /
field_value_factor query in ElasticRequestHandler works unchanged against the
grouped structure.

Feature toggle

Guarded by FT_OAK-12353, enabled by default. Set to false at runtime to revert
to the previous one-nested-document-per-value behavior.

A time-bombed test (ElasticDocumentTest#ft_oak_12353_toggleShouldBeRemoved) will start
failing after 2027-08-12 as a reminder to remove the toggle and its guards once the
grouped format has been running in production long enough.

Changes

• ElasticDocument: introduces the FT_OAK-12353 toggle; groups dynamic boost values
  by boost score in addDynamicBoostField/getProperties when enabled.
• ElasticIndexProviderService: registers the toggle on the OSGi Whiteboard.
• Tests: unit coverage for the grouping logic in ElasticDocumentTest, and end-to-end
  query coverage (grouped and ungrouped) in ElasticDynamicBoostTest.

Test plan

• ElasticDocumentTest — verifies grouping by boost, single-group unwrapping, and
  toggle-disabled fallback to the original per-value nested documents
• ElasticDynamicBoostTest — end-to-end query tests against a real Elasticsearch
  instance, confirming queries still match on values grouped into a shared nested
  document, both with grouping enabled (default) and disabled