[Enh]: Include fields in `describe_entities` from metadata when not in `fields[]`.

## What?

Today fields are only included in `describe_entities` when the `fields[]` property includes them.

This enhancement includes fields (based on the role) in `describe_entities` from `fields[]` as the primary source and the database metadata as the secondary source.

This ensures every field is always included in `describe_entities`.

(internal reference: tribat)

Start at the describe_entities entry point and trace how fields[] and database metadata are combined for the requested role. Verify the behavior for fields present only in metadata; done means every role-available field is included, with fields[] taking precedence over metadata.

Written by the indexing model from the issue text.