ESQL: Fix filter pushdowns for union types

[alex-spies]

Related to #127521 and, in the same vein, the LucenePushdownPredicates don't use FieldAttributes#fieldName here but just the name method. For field attributes representing converted union types, like $$field$converted_to$long the actual field name (field) is different from the attribute name, which means that this doesn't correctly apply to union types.

I tried to quickly fix this but this regresses the csv tests for union types, which is consistent with some observations by @fang-xing-esql in #127797 that union types don't work correctly with attribute pushdowns.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[alex-spies]

I realized that in our TranslatorHandler and related code, we use FieldAttribute#name rather than FieldAttribute#fieldName in multiple places. This will prevent us from emitting correct queries when pushing down predicates for union types.

Examples:

• wrapFunctionQuery
• TranslatorHandler#nameOf

I think a nice way of preventing this would be to update the classes in the querydsl package, like SingleValueQuery, and make them require a FieldName object rather than a String. This would mostly fix this part of the issue at compile time.