Add preserve_leaf_arrays, properties, and passthrough to FlattenedProperty

[MattDevy]

Summary

Adds three missing fields to FlattenedProperty in specification/_types/mapping/complex.ts.

• preserve_leaf_arrays?: 'lossy' | 'exact' — controls how leaf arrays are reconstructed in synthetic _source (Add option to enable accurate leaf arrays for flattened fields elasticsearch#145376, merged 2026-04-07)
• properties?: Dictionary<PropertyName, Property> — maps specific keys within the flattened object as typed sub-fields (Add sub-field support to flattened field type elasticsearch#144451, merged 2026-03-26)
• passthrough?: FlattenedPropertyPassthrough — makes typed sub-fields queryable at root level without the flattened field prefix; priority resolves naming conflicts (Add passthrough behavior for FlattenedFieldMapper mapped sub-fields elasticsearch#145131, merged 2026-04-01)

All three are @availability stack since=9.4.0 stability=stable. A new FlattenedPropertyPassthrough class is added to hold the { priority: integer } object shape.

Test plan

• Verify generated schema includes all three new fields on FlattenedProperty
• Verify FlattenedPropertyPassthrough appears in the schema with required priority: integer
• Verify @availability stack since=9.4.0 stability=stable is reflected in output for all three fields
• Confirm fields are absent from serverless schema (stack-only annotations)

[github-actions]

Following you can find the validation changes against the target branch for the API.

API	Status	Request	Response
indices.create	🔴	1521/1587 → 1524/1587	1587/1587

You can validate this API yourself by using the make validate target.

[l-trotta]

wait where's properties?

[MattDevy]

wait where's properties?

I added properties, then removed it as it clashes with the "properties" key name / the compiler freaked out.

Let me put this one back in draft, might need to work out how to re-add it properly