Harden Elasticsearch indexing and search for larger module corpora

[jloh02]

Problem

The timeout increase in #4463 is an immediate mitigation, but the scraper/search integration still has several size-related assumptions that are now brittle with the current corpus size.

Observed production context:

• modules_v2 has about 21,096 documents.
• moduleInfo bulk indexing is sent as one large request, about 18.5 MB.
• ElasticPersist.getModuleCodes() fetches at most 20,000 docs in a single search.
• modules_v2 has index.max_result_window = 20000.
• Frontend pagination/random-course paths can request beyond the ES result window and receive HTTP 400.
• createIndex() assumes a specific error shape and can throw Cannot read properties of undefined (reading 'type') for other ES errors.

Proposed work

• Chunk scraper moduleInfo bulk writes by document count and/or byte size.
• Add retry/backoff for transient ES timeout/connection failures.
• Replace getModuleCodes() single size: 20000 search with scroll/search-after so cleanup sees all existing documents.
• Fix frontend deep pagination/random-course behavior so it does not request beyond index.max_result_window, or move those paths to a cursor/search-after-safe design.
• Harden ES error handling in createIndex() and delete paths so unexpected ES/network errors are logged clearly without masking the original cause.

Notes

This is intentionally separate from the timeout mitigation PR so the production scraper can be unblocked first.