AO3-7284 Configurable indexer batch size

[not-varram]

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Issue

https://otwarchive.atlassian.net/browse/AO3-7284

Purpose

Make Search::Indexer batch size configurable via config/config.yml instead of hardcoded.

Testing Instructions

1. Set SEARCH_INDEXER_BATCH_SIZE in config/config.yml (default is 1000).
2. Run:
   • docker compose run --rm test bundle exec rspec spec/models/search/indexer_spec.rb spec/lib/tasks/search_tasks.rake_spec.rb
3. Optional functional check:
   • docker compose run --rm test bundle exec rake search:reindex_collections
4. Confirm all commands pass.

Credit

varram (he/him)

[Bilka2]

Thanks for working on this, I have a few comments

[Bilka2]

Suggested change

	# Batch size used by Search::Indexer when reading records from the database:
	# Batch size used by Indexer when reading records from the database:

[Bilka2]

This is always set since it's in the config, so there's no reason to have a default here

[Bilka2]

We don't normally stub like this, could you please work with less stubbing/mocking here? It would be better to actually make records and then index those, e.g. make a few works and then call the method on the WorkIndexer. Then the only thing you have to stub/mock is the AsyncIndexer, so you're interacting a lot less with the innards of the class you're testing