Fix pipeline robustness and MinerU content compatibility

[huiserwang]

Summary

• tolerate null country values from OpenAlex during legacy export
• add gemini-3.6-flash-search and adjust Search/Light LLM timeouts
• fix Search LLM timeout exception handling
• normalize both flat and page-grouped MinerU content_list formats for new results and existing caches
• add regression coverage for API null values and MinerU response variants

Testing

• 144 passed
• 1 failed: pre-existing CitingDescriptionCache persistence-contract test (test_persists_to_disk), unrelated to this PR
• targeted MinerU and pipeline tests pass
• compile and diff checks pass