test: drive the production hybrid chunking loop with a configured chunk size

Problem

The configurable hybrid backend page batch size feature (--hybrid-chunk-size, branch feat/configurable-hybrid-chunk-size) threads HybridConfig.chunkSize into HybridDocumentProcessor.effectiveChunkSize, but the production chunking loop (HybridDocumentProcessor.java:658) has zero direct test coverage: the loop is verified only through a test-side mirror (splitIntoChunks). A regression in the real loop (wrong config field read, off-by-one in subList bounds) would pass CI.

Suggested fix

Add a test that builds a Config/HybridConfig with non-default chunkSize (e.g. 10), drives the actual backend path (mock HybridClient), and asserts requests are batched at 10 pages; replace the splitIntoChunks mirror with coverage of the real loop.

Severity: P2 | Confidence: 75 | Reviewer(s): testing