fix(retain): preserve JSON chunks during output retry

[koriyoshi2041]

Summary

• split OutputTooLong retry chunks at conversation JSON-array boundaries before falling back to text splitting
• split a single oversized conversation turn inside its content field while rewrapping it as a valid JSON array
• drop unsplittable sub-chunks instead of recursively retrying without progress

Refs #2571.

Tests

• uv run pytest hindsight-api-slim/tests/test_fact_extraction_retry.py -q
• uv run ruff check hindsight-api-slim/hindsight_api/engine/retain/fact_extraction.py hindsight-api-slim/tests/test_fact_extraction_retry.py
• uv run ruff format --check hindsight-api-slim/hindsight_api/engine/retain/fact_extraction.py hindsight-api-slim/tests/test_fact_extraction_retry.py
• pre-commit hook: check-unused.sh, generate-docs-skill.sh, lint.sh

[benfrank241]

lgtm, thanks!