ES|QL Access All Ingested Data: Query fields that were never mapped (GA)

## Value Proposition

Teams ingest data from dozens of sources, and not every field is mapped at index time. Today, unmapped fields are invisible to queries — analysts don't know what they're missing, and the only fix is to update the mapping and reindex. Access All Ingested Data (schema-on-read) lets ES|QL query any field present in `_source`, whether mapped or not, without touching mappings or reindexing.

## Expected Outcome

Schema-on-read shipped in Tech Preview in Elasticsearch 9.4 with two modes: `SET unmapped_fields="load"` (infer types from `_source`) and `SET unmapped_fields="nullify"` (treat unmapped as null).

For GA, the remaining work focuses on correctness and hardening:

- Consistent behavior when LOAD/NULLIFY interacts with branching commands (FORK, subqueries, views)
- Correct handling of partially mapped fields (fields mapped in some indices but not others)
- Full-text search (MATCH) working alongside LOAD on mapped fields
- Late materialization of nullified fields