OAK-12248: graceful 404 when ES alias is not yet provisioned

[bhabegger]

Summary

• Adds feature toggle FT_OAK-12248 (off by default) to ElasticIndexStatistics
• When enabled, a 404 ElasticsearchException from an ES count request during query planning is caught in getCountOrZeroOn404(), logged at INFO, and treated as 0 estimated documents — the planner stays on the ES path instead of falling back to traversal with an ERROR log
• ElasticResultRowAsyncIterator.onFailure() / hasNext() also check the toggle to suppress ERROR at execution time if the alias is still missing
• Toggle registered in ElasticIndexProviderService alongside the existing FT_OAK-12206 and FT_OAK-12234 toggles

Root cause note

The 404 first surfaces during planning (not execution): ElasticIndexStatistics.getDocCountFor() fires an ES count request whose ElasticsearchException propagates uncaught to FulltextIndex.getPlans():176 and is logged as ERROR there. Oak's cache re-throws RuntimeException causes unwrapped (only checked exceptions are wrapped in CompletionException), so the catch must target ElasticsearchException directly.

Test

ElasticGraceful404QueryTest (new):

• queryOnMissingAlias_withToggleOff_logsError — seeds data, provisions index, deletes alias, asserts ERROR log (confirms existing behaviour)
• queryOnMissingAlias_withToggleOn_returnsEmptyWithoutError — same setup, toggle on, asserts 0 results + no ERROR + INFO "alias not found"

Relation to other tickets

Builds on OAK-12247 (track totalIndexedNodes). Prerequisite for OAK-12249 (lazy ES provisioning): without this toggle, a non-existent alias after an empty reindex would ERROR on every query.

🤖 Generated with Claude Code

[nit0906]

@bhabegger - do we need a toggle for this ?

The change seems very limited.

[bhabegger]

@bhabegger - do we need a toggle for this ?

The change seems very limited.

Yes, that's why it is on by default. This is just a precaution because it does change the behavior (maybe in some situations we would want an error if the index doesn't exist ?).