feat: parallelize text and multimodal processing in process_document_complete

[ndcorder]

Text insertion and multimodal processing in process_document_complete run sequentially right now but don't share any state. This runs them concurrently with asyncio.gather so total time is max(text, multimodal) instead of text + multimodal.

Uses return_exceptions=True so if one branch fails the other still completes.

[LarFii]

Thanks for your contribution!

I found one P1 issue. In raganything/processor.py, the new parallel branch uses asyncio.gather(..., return_exceptions=True) and only logs per-task failures. That means process_document_complete() no longer propagates text/multimodal failures, so the outer error handler is skipped and the method can still emit on_document_complete / "processing complete" even when one branch failed. This is a behavioral regression from main and can silently report partially ingested documents as successful. I think the PR should preserve fail-fast semantics here, or at minimum aggregate task errors and raise after both tasks finish. The new test test_one_branch_failing_does_not_block_the_other currently locks in the broken behavior rather than detecting it.