fix(retain): honour the retain strict-schema flag in the batch path (#3656)

The batch retain path chose the schema builder from the global llm_strict_schema
but sent strict: from the retain-scoped llm_strict_schema_retain, so the two
could disagree. Resolve the flag once and use it for both, matching what the
providers and the streaming retain path already do.

Co-authored-by: Kamil Furtak <frtkkamil@icloud.com>